Method for the preparation of 2,4-dihydroxybutyrate

ABSTRACT

A method for the preparation of 2,4-dihydroxybutyric acid from homoserine includes a first step of conversion of the primary amino group of homoserine to a carbonyl group to obtain 2-oxo-4-hydroxybutyrate, and a second step of reduction of the obtained 2-oxo-4-hydroxybutyrate (OHB) to 2,4-dihydroxybutyrate.

The present invention relates to a novel method for the preparation of 2,4-dihydroxybutyrate (2,4-DHB) from homoserine comprising a two-step pathway:

-   -   a first step of conversion of the primary amino group of         homoserine to a carbonyl group to obtain         2-oxo-4-hydroxybutyrate, and     -   a second step of reduction of the obtained         2-oxo-4-hydroxybutyrate (OHB) to obtain 2,4-DHB.

The carboxylic acids cited within the present application are equally named under their salt (e.g. 2,4-dihydroxyburyrate) or acid forms (e.g. 2,4-dihydroxybutyric acid).

2,4-dihydroxybutyric acid (equally 2,4-DHB or DHB) is a compound of considerable economic interest. DHB can be readily converted into α-hydroxy-γ-butyrolactone in aqueous media by adjusting the appropriate pH. α-hydroxy-γ-butyrolactone is a prominent precursor for the production of the methionine substitute 2-hydroxy-4-(methylthio)-butyrate (HMTB) (US 2009/318715) which has a large market in animal nutrition. At present, α-hydroxy-γ-butyrolactone is derived from γ-butryolactone by a multi-stage process that implies halogenation of the γ-butryolactone in position α, and subsequent substitution of the halogen atom by a hydroxyl group in alkaline medium (US 2009/318715).

From growing oil prices the need for the production of DHB from renewable resources arises. Microorganisms are capable of transforming biomass-derived raw material, e.g. sugars or organic acids, into a large variety of different chemical compounds (Werpy & Petersen, 2004). With the growing body of biochemical and genomic information it is possible to modify microorganisms such that they overproduce naturally occurring metabolic intermediates with high yield and productivity (Bailey, 1991). Optimization of production microorganisms often requires rational engineering of metabolic networks which ensures, among others, overexpression of enzymes required for the biosynthesis of the metabolite of interest, and alleviation of product feedback inhibition. Another possibility is the implementation of novel enzymatic systems that catalyze the production of a metabolite of interest.

Metabolic engineering approaches and enzymatic catalyses require detailed knowledge of the biochemistry and regulation of the metabolic pathway leading to the metabolite of interest. In the case of DHB production, this information is not available. Only few studies report the occurrence of DHB in patients with succinic semialdehyde dehydrogenase deficiency (Shinka et al., 2002) without, however, identifying enzymatic reactions implicated in DHB production. The zymotic or enzymatic production of DHB, therefore, requires (i) the identification of a thermodynamically feasible pathway which transforms an accessible precursor into DHB, (ii) the identification or construction of enzymes that are capable to catalyze individual reaction steps in the pathway and (iii) the functional expression of the pathway enzymes in an appropriate production organism. The present invention has as an objective to satisfy these needs.

Accordingly, one object of the present invention is a method of preparation of 2,4-DHB from homoserine comprising a two-step pathway (see FIG. 1):

-   -   a first step of conversion of the primary amino group of         homoserine to a carbonyl group to obtain OHB, and     -   a second step of reduction of the obtained OHB to 2,4-DHB.

The first and/or the second step(s) of the method of the invention can be catalyzed either by an enzyme encoded by an endogenous or a heterologous gene.

In the description, enzymatic activities are also designated by reference to the genes coding for the enzymes having such activity. The use of the denomination of the genes is not limited to a specific organism, but covers all the corresponding genes and proteins in other organisms (e.g. microorganisms, functional analogues, functional variants and functional fragments thereof as long as they retain the enzymatic activity).

Within a further aspect of the invention, the enzyme converting the primary amino group of homoserine to a carbonyl group to obtain OHB can be homoserine transaminase, homoserine dehydrogenase, or homoserine oxidase.

Within a further aspect of the invention, the enzyme having homoserine transaminase activity can be identified among enzymes having aspartate transaminase (EC2.6.1.1) activity, branched-chain-amino-acid transaminase (EC2.6.1.42) activity, or aromatic-amino-acid transaminase (EC2.6.1.57) activity.

Within a further aspect of the invention, the homoserine transaminase can be the branched-chain-amino-acid transaminase from Escherichia coli, Ec-IlvE, and Lactococcus lactis, Ll-BcaT, the aromatic-amino-acid transaminases from E. coli, Ec-TyrB, L. lactis, LI-AraT, and Saccharomyces cerevisiae, Sc-Aro8, or the aspartate transaminase from E. coli, Ec-AspC.

The second step of the method of the present invention is catalysed by an enzyme having OHB reductase activity. Within a further aspect of the invention, the enzyme having OHB reductase activity can be identified among enzymes having 2-hydroxyacid dehydrogenase activity, in particular among enzymes having lactate dehydrogenase (Ldh) (EC1.1.1.27, EC1.1.1.28), malate dehydrogenase (Mdh) (EC1.1.1.37, EC1.1.1.82, EC1.1.1.299) activity, or branched chain (D)-2-hydroxyacid dehydrogenase (EC1.1.1.272, EC1.1.1.345) activity. More specifically, the enzyme having homoserine transaminase activity is encoded by genes ilvE, tyrB, aspC, araT, bcaT, or ARO8.

In an even more specific aspect, the enzyme having homoserine transaminase activity is encoded by sequence set forth in SEQ ID No.59, SEQ ID No.61, SEQ ID No.63, SEQ ID No.65, SEQ ID No. 67 or SEQ ID No.69 or any sequence sharing a homology of at least 50% with said sequences or corresponds to SEQ ID No.60, SEQ ID No.62, SEQ ID No.64, SEQ ID No.66, SEQ ID No.68, SEQ ID No.70 or any sequence sharing a homology of at least 50% with said sequences.

Within a further aspect of the invention, the OHB reductase enzyme can be the (L)-lactate dehydrogenase from Lactococcus lactis (Ll-LdhA), from Oryctalagus cuniculus (Oc-LldhA), from Geobacillus stearothermophilus (Gs-Lldh), or from Bacillus subtilis (Bs-Ldh) , the (D)-lactate dehydrogenase from Escherichia coli (Ec-LdhA), the (L)-malate dehydrogenase from Escherichia coli (Ec-Mdh), or the branched chain (D)-2-hydroxyacid dehydrogenase from Lactococcus lactis (Ll-PanE).

In an even more specific aspect of the invention the OHB reductase enzyme is represented by the amino acid sequences SEQ ID No. 2, SEQ ID No. 4, SEQ ID No. 6, SEQ ID No. 8, SEQ ID No. 10, SEQ ID No. 12, SEQ ID No. 14, SEQ ID No. 288, SEQ ID No. 30, SEQ ID No. 32, SEQ ID No. 102, SEQ ID No. 104, SEQ ID No. 106, SEQ ID No. 108, SEQ ID No. 110, SEQ ID No. 112, SEQ ID No. 114, SEQ ID No. 116 or SEQ ID No. 118 or any sequence sharing a homology of at least 50% with said sequences, or is encoded by the nucleic acid sequences represented by SEQ ID No.1, SEQ ID No. 3, SEQ ID No. 5, SEQ ID No. 7, SEQ ID No. 9, SEQ ID No. 11, SEQ ID No. 13, SEQ ID No. 287, SEQ ID No. 29, SEQ ID No. 31, SEQ ID No. 101, SEQ ID No. 103, SEQ ID No. 105, SEQ ID No. 107, SEQ ID No. 109, SEQ ID No.111, SEQ ID No. 113, SEQ ID No. 115 or SEQ ID No. 117 or any sequence sharing a homology of at least 50% with said sequences.

In a further aspect, the invention also deals with the use of an enzyme reducing OHB to 2,4-DHB as above described.

Proteins sharing substantial homology with the above enzymes are also another aspect of the invention such as functional variants or functional fragments.

The expression “substantial homology” covers homology with respect to structure and/or amino acid components and/or biological activity.

More generally, within the meaning of the invention the homology between two protein sequences can be determined by methods well known by the skilled man in the art. It is generally defined as a percentage of sequence identity between a reference sequence and the sequence of a protein of interest.

As used herein, “percent (%) sequence identity” with respect to the amino acid or nucleotide sequences identified herein is defined as the percentage of amino acid residues or nucleotides in a candidate sequence that are identical with the amino acid residues or nucleotides in an enzyme sequence, after aligning the sequences and introducing gaps, if necessary, to achieve the maximum percent sequence identity, and not considering any conservative substitutions as part of the sequence identity. Methods for performing sequence alignment and determining sequence identity are known to the skilled artisan, may be performed without undue experimentation, and calculations of identity values may be obtained with definiteness. See, for example, Ausubel, et al., eds. (1995) Current Protocols in Molecular Biology, Chapter 19 (Greene Publishing and Wiley-Interscience, New York); and the ALIGN program (Dayhoff (1978) in Atlas of Protein Sequence and Structure 5:Suppl. 3 (National Biomedical Research Foundation, Washington, D.C.). A number of algorithms are available for aligning sequences and determining sequence identity and include, for example, the homology alignment algorithm of Needleman et al. (1970) J. Mol. Biol. 48:443; the local homology algorithm of Smith, et al. (1981) Adv. Appl. Math. 2:482; the search for similarity method of Pearson, et al. (1988) Proc. Natl. Acad. Sci. 85:2444; the Smith-Waterman algorithm (Meth. Mol. Biol. 70:173-187 (1997); and BLASTP, BLASTN, and BLASTX algorithms (see Altschul, et al. (1990) J. Mol. Biol. 215:403-410). Computerized programs using these algorithms are also available, and include, but are not limited to: ALIGN or Megalign (DNASTAR) software, or WU-BLAST-2 (Altschul, et al., Meth. Enzym., 266:460-480 (1996)); or GAP, BESTFIT, BLAST (Altschul, et al.), supra, FASTA, and TFASTA, available in the Genetics Computing Group (GCG) package, Version 8, Madison, Wis., USA; and CLUSTAL in the PC/Gene program by Intelligenetics, Mountain View, Calif. Those skilled in the art can determine appropriate parameters for measuring alignment, including algorithms needed to achieve maximal alignment over the length of the sequences being compared. Preferably, the sequence identity is determined using the default parameters determined by the program. Specifically, sequence identity can be determined by the Smith-Waterman homology search algorithm (Meth. Mol. Biol. 70:173-187 (1997)) as implemented in MSPRCH program (Oxford Molecular) using an affine gap search with the following search parameters: gap open penalty of 12, and gap extension penalty of 1. Preferably, paired amino acid comparisons can be carried out using the GAP program of the GCG sequence analysis software package of Genetics Computer Group, Inc., Madison, Wis., employing the blosum 62 amino acid substitution matrix, with a gap weight of 12 and a length weight of 2. With respect to optimal alignment of two amino acid sequences, the contiguous segment of the variant amino acid sequence may have additional amino acid residues or deleted amino acid residues with respect to the reference amino acid sequence. The contiguous segment used for comparison to the reference amino acid sequence will include at least 20 contiguous amino acid residues, and may be 30, 40, 50, or more amino acid residues. Corrections for increased sequence identity associated with inclusion of gaps in the derivative's amino acid sequence can be made by assigning gap penalties.

The enzymes according to the present invention having the same activity (either OHB reductase, or the enzyme converting the primary amino group of homoserine to a carbonyl group to obtain OHB) share at least about 50%, 70% or 85% amino acid sequence identity, preferably at least about 85% amino acid sequence identity, more preferably at least about 90% amino acid sequence identity, even more preferably at least about 95% amino acid sequence identity and yet more preferably 98% amino acid sequence identity. Preferably, any amino acid substitutions are “conservative amino acid substitutions” using L-amino acids, wherein one amino acid is replaced by another biologically similar amino acid. Conservative amino acid substitutions are those that preserve the general charge, hydrophobicity/hydrophilicity, and/or steric bulk of the amino acid being substituted. Examples of conservative substitutions are those between the following groups: Gly/Ala, Val/Ile/Leu, Lys/Arg, Asn/Gln, Glu/Asp, Ser/Cys/Thr, and Phe/Trp/Tyr. A derivative may, for example, differ by as few as 1 to 10 amino acid residues, such as 6-10, as few as 5, as few as 4, 3, 2, or even 1 amino acid residue.

The term functional variant encompasses enzymes that may present substantial sequence modifications when compared to the sequences specifically described within the present application but that still retain the original enzymatic activity.

It also means that the sequence of the enzyme may comprise less amino acids than the original one but said truncated enzyme still retains the original enzymatic activity.

According to an aspect of the invention, the activity of the enzyme catalyzing the first and/or the second step of the method of the present invention is enhanced. This enhancement can be measured by an enzymatic assay as described in Examples 1 or 4.

Improvement of said enzymes can be obtained by at least one mutation, said mutation(s) (i) improving the activity and/or substrate affinity of the mutated enzyme for homoserine or OHB respectively, and or (ii) decreasing the activity and/or substrate affinity of the mutated enzyme for their natural substrate.

Within the present invention, the expression “improve the activity and/or substrate affinity” means that the enzyme before mutation, was either

-   -   unable to use the substrate, and/or     -   synthesized the product of the reaction at a maximum specific         rate at least three times lower, and/or     -   had an affinity for homoserine or OHB that was at least three         times lower, and/or.     -   had a maximum specific activity on the natural substrate that         was at least three times higher, and/or.     -   had an affinity for the natural substrate that was at least         three times higher.

In a still further aspect the invention encompasses the nucleotide sequences encoding the enzymes catalyzing the first and the second step of the method of the invention.

In an even more specific aspect of the invention the OHB reductase enzyme is encoded by the nucleic acid sequences represented by SEQ ID No.1, SEQ ID No. 3, SEQ ID No. 5, SEQ ID No. 7, SEQ ID No. 9, SEQ ID No. 11, SEQ ID No. 13, SEQ ID No. 287, SEQ ID No. 29, SEQ ID No. 31, SEQ ID No. 101, SEQ ID No. 103, SEQ ID No. 105, SEQ ID No. 107, SEQ ID No. 109, SEQ ID No.111, SEQ ID No. 113, SEQ ID No. 115 or SEQ ID No. 117 or any sequence sharing a homology of at least 50% with said sequences.

The OHB reductase according to the invention corresponds in a specific aspect to (L)-lactate dehydrogenase A comprising at least one mutation when a compared to the wild type enzyme in at least one of the positions V17, Q85, E89, I226, or A222. These positions are conserved in the lactate dehydrogenase family, and they are defined in this text by reference to the Lactococcus lactis (L)-lactate dehydrogenase A (SEQ ID No. 6). The skilled man in the art will then easily identify the corresponding amino acid residues in other lactate dehydrogenases by an alignment of the corresponding amino acid sequences. Therefore, the invention also provides for changes of these amino acids in other lactate dehydrogenase enzymes.

The OHB reductase according to the invention corresponds in a specific aspect to (L)-malate dehydrogenase comprising at least one mutation when compared to the wild type enzyme in at least one of the positions 112, R81, M85, D86, V93, G179, T211, or M227. These positions are conserved in the malate dehydrogenase family, and they are defined in this text by reference to the sequence of the E. coli (L)-malate dehydrogenase (SEQ ID No. 2). The man skilled in the art will easily identify the corresponding amino acid residues in other malate dehydrogenases by an alignment of the corresponding amino acid sequences. Therefore, the invention also provides for changes of these amino acids in other malate dehydrogenase enzymes.

In accordance with this invention, a “nucleic acid sequence” refers to a DNA or RNA molecule in single or double stranded form, preferably a DNA molecule. An “isolated DNA”, as used herein, refers to a DNA which is not naturally-occurring or no longer in the natural environment wherein it was originally present, e.g., a DNA coding sequence associated with other regulatory elements in a chimeric gene, a DNA transferred into another host cell, or an artificial, synthetically-made DNA sequence having a different nucleotide sequence compared to any naturally-occurring DNA sequence.

The present invention also relates to a chimeric gene comprising, functionally linked to one another, at least one promoter which is functional in a host organism, a polynucleotide encoding anyone of the enzymes catalyzing first and second step of the method as defined according to the invention, and a terminator element that is functional in the same host organism. The various elements which a chimeric gene may contain are, firstly, elements regulating transcription, translation and maturation of proteins, such as a promoter, a sequence encoding a signal peptide or a transit peptide, or a terminator element constituting a polyadenylation signal and, secondly, a polynucleotide encoding a protein. The expression “functionally linked to one another” means that said elements of the chimeric gene are linked to one another in such a way that the function of one of these elements is affected by that of another. By way of example, a promoter is functionally linked to a coding sequence when it is capable of affecting the expression of said coding sequence. The construction of the chimeric gene according to the invention and the assembly of its various elements can be carried out using techniques well known to those skilled in the art. The choice of the regulatory elements constituting the chimeric gene depends essentially on the host organism in which they must function, and those skilled in the art are capable of selecting regulatory elements which are functional in a given host organism. The term “functional” is intended to mean capable of functioning in a given host organism.

The promoters which the chimeric gene according to the invention may contain are either constitutive or inducible. By way of example, the promoters used for expression in bacteria may be chosen from the promoters mentioned below. For expression in Escherichia coli mention may be made of the lac, trp, lpp, phoA, recA, araBAD, prou, cst-I, tetA, cadA, nar, tac, trc, lpp-lac, Psyn, cspA, PL, PL-9G-50, PR-PL, T7, [lambda]PL-PT7, T3-lac, T5-lac, T4 gene 32, nprM-lac, VHb and the protein A promoters or else the Ptrp promoter (WO 99/64607). For expression in Gram-positive bacteria such as Corynebacteria or Streptomyces, mention may be made of the PtipA or PS1 and PS2 (FR91/09870) promoters or those described in application EP0629699A2. For expression in yeasts and fungi, mention may be made of the K. lactis PLAC4 promoters or the K. lactis Ppgk promoter (patent application FR 91/05294), the Trichoderma reesei tef1 or cbh1 promoter (WO 94/04673), the Penicillium funiculosum his, csl or apf promoter (WO 00/68401) and the Aspergillus niger gla promoter.

According to the invention, the chimeric gene may also comprise other regulatory sequences, which are located between the promoter and the coding sequence, such as transcription activators (enhancers).

As such, the chimeric gene of the invention comprises in a specific embodiment at least, in the direction of transcription, functionally linked, a promoter regulatory sequence which is functional in a host organism, a nucleic acid sequence encoding a polynucleotide encoding anyone of the enzymes catalyzing first and second step of the method as defined according to the invention and a terminator regulatory sequence which is functional in said host organism.

The present invention also relates to a cloning and/or expression vector comprising a chimeric gene according to the invention or a nucleic acid sequence of the invention. The vector according to the invention is of use for transforming a host organism and expressing in this organism anyone of the enzymes catalyzing the first and/or the second step(s) of the method of the present invention. This vector may be a plasmid, a cosmid, a bacteriophage or a virus. Preferentially, the transformation vector according to the invention is a plasmid. Generally, the main qualities of this vector should be able to maintain itself and to self-replicate in the cells of the host organism, in particular by virtue of the presence of an origin of replication, and to express anyone of the enzymes catalyzing the first and/or the second step(s) of the method of the present invention therein. For the purpose of stable transformation of a host organism, the vector may also integrate into the genome. The choice of such a vector, and also the techniques of insertion of the chimeric gene according to the invention into this vector are part of the general knowledge of those skilled in the art. Advantageously, the vector used in the present invention also contains, in addition to the chimeric gene according to the invention, a chimeric gene encoding a selectable marker. This selectable marker makes it possible to select the host organisms which are effectively transformed, i.e. those which incorporated the vector. According to a particular embodiment of the invention, the host organism to be transformed is a bacterium, a yeast, a fungus. Among the selectable markers which can be used, mention may be made of markers containing genes for resistance to antibiotics, such as, for example, the hygromycinphosphotransferase gene. Other markers may be genes to complement an auxotrophy, such as the pyrA, pyrB, pyrG, pyr4, arg4, argB and trpC genes, the molybdopterin synthase gene or that of acetamidase. Mention may also be made of genes encoding readily identifiable enzymes such as the GUS enzyme, or genes encoding pigments or enzymes regulating the production of pigments in the transformed cells. Such selectable marker genes are in particular described in patent applications WO 91/02071, WO 95/06128, WO 96/38567 and WO 97/04103.

The present invention also relates to modified microorganisms.

More specifically, the modified microorganism of the invention allows the preparation of 2,4-DHB from homoserine by a two-step pathway comprising:

-   -   a first step of conversion of the primary amino group of         homoserine to a carbonyl group to obtain         2-oxo-4-hydroxybutyrate, and     -   a second step of reduction of the obtained         2-oxo-4-hydroxybutyrate to obtain 2,4-dihydroxybutyrate.

The enzymes involved in the two steps are those above described.

The term “microorganism” is intended to mean any lower unicellular organism into which the chimeric gene(s), nucleic acid(s) or vector(s) according to the invention may be introduced in order to produce 2,4-DHB. Preferably, the host organism is a microorganism, in particular a fungus, for example of the Penicillium, Aspergillus and more particularly Aspergillusflavus, Chrysosporium or Trichoderma genus, a yeast, in particular of the Saccharomycetaceae, Pichiaceae or Schizosaccharomycetaceae, most preferentially Saccharomyces cerevisiae, Schizosaccharomyces pombe, Kluyveromyces lactis, Kluyveromyces marxianus, or Pichia jadinii, Pichia stipitis or Pichia pastoris, a bacterium, preferentially selected among Enterobacteriaceae, Clostridiaceae, Bacillaceae, Streptomycetaceae, Streptococcaceae, Methylobacteriacae, and Corynebacteriaceae, most preferentially Escherichia coli, Bacillus subtilis, Corynebacterium glutamicum, Clostridium acetobutylicum, Methylobacterium extorquens or Lactococcus lactis.

The present invention also relates to modified microorganisms containing at least one chimeric gene according to the invention, either integrated into their genome or carried on an extra-chromosomal genetic element, for example a plasmid. In a more specific aspect of the invention, the transformed host organism comprises a nucleic acid of the invention encoding a polypeptide converting the primary amino acid group of homoserine to a carbonyl group to obtain OHB and/or a nucleic acid encoding a polypeptide reducing OHB in 2,4-DHB or a chimeric gene comprising a nucleic acid encoding a polypeptide converting the primary amino acid group of homoserine to a carbonyl group to obtain OHB, and/or a OHB reductase or an expression vector comprising a nucleic acid encoding a polypeptide converting the primary amino acid group of homoserine to a carbonyl group to obtain OHB, or a polypeptide having a OHB reductase activity.

Within a further aspect of the invention, the synthetic pathway for the conversion of homoserine into DHB is expressed in a microorganism with enhanced production of homoserine. Enhanced production of homoserine in microorganisms can be achieved by (i) overexpressing the enzymes aspartate kinase, aspartate semialdehyde dehydrogenase, and homoserine dehydrogenase, (ii) by rendering the aspartate kinase enzyme insensitive to product inhibition that can be brought about by lysine, methionine, or threonine, and (iii) by deletion of metabolic pathways that branch off the homoserine biosynthesis pathway. Overexpression of aspartate kinase, aspartate semialdehyde dehydrogenase, and homoserine dehydrogenase can be achieved by expressing the enzymes from a multicopy plasmid under the control of an appropriate constitutive or inducible promoter. Alternatively, overexpression of said enzymes can be achieved by deletion of transcriptional repressors that limit the transcription of genes coding for aspartate kinase, aspartate semialdehyde dehydrogenase, and homoserine dehydrogenase. Aspartate kinases can be rendered insensitive to inhibition by aspartate-derived amino acids by introducing appropriate mutations into their amino acid sequences. Entry points into metabolic pathways that branch off the homoserine biosynthesis pathway are catalyzed by enzymes having O-succinyl homoserine or O-acetyl homoserine synthase activity (entry into methionine biosynthesis), homoserine kinase activity (entry into threonine biosynthesis), or diaminopimelate decarboxylase activity (entry into lysine biosynthesis). Deletion of genes encoding proteins having said enzymatic activities avoids formation aspartate-derived amino acids and therefore aids homoserine formation.

Accordingly, deletion of the genes metA, thrB, and lysA in E. coli attenuates pathways that branch of the homoserine biosynthetic pathway. The increase of enzymatic activities of the homoserine pathway in E. coli can be achieved, for instance, by the overexpression of the bifunctional aspartate kinase-homoserine dehydrogenase mutant thrA S345F (insensitive to threonine inhibition) and asd (both genes from E. coli); or by the overexpression of the monofunctional aspartate kinase mutant lysC E250K (insensitive to lysine), asd (both genes from E. coli), and the homoserine dehydrogenase gene HOM6 from S cerevisiae.

The microorganism of the invention may also have attenuated capacity to export homoserine which increases the intracellular availability of this amino acid. In order to achieve decreased homoserine export from the cells, permeases capable of exporting homoserine can be deleted. Such permeases may be identified by overexpressing genomic libraries in the microorganism and cultivating said microorganism at inhibitory concentrations of homoserine or structurally similar amino acids such as threonine, leucine, or aspartate (Zakataeva et al. 1999/FEBS Lett/452/228-232). Genes whose overexpression confers growth at increased concentrations of either of said amino acids are likely to participate in homoserine export.

In a further aspect, the microorganism of the invention being Escherichia coli carries deletions in the homoserine efflux transporters rhtA, rhtb, and/or rhtC.

Efficient production of DHB can be ensured by optimizing carbon flux repartitioning in the metabolic network of the host organism with respect to the optimization of cofactor supply for DHB synthesis, and attenuation of competing pathways that cause formation of metabolic by-products other than DHB. An important tool for strain improvement provides constraint-based flux balance analysis. This method allows calculating the theoretical yield of a given metabolic network depending on cultivation conditions, and facilitates identification of metabolic targets for overexpression or deletion. The experimental techniques used for overexpression and deletion of the metabolic target reaction are described (Example 8).

Accordingly, the microorganism of the invention may also exhibit enzymatic activities chosen among phosphoenolpyruvate carboxylase, phosphoenolpyruvate carboxykinase, isocitrate lyase, pyruvate carboxylase, and hexose symporter permease which is increased, and/or at least one of the enzymatic activities chosen among lactate dehydrogenase, alcohol dehydrogenase, acetate kinase, phosphate acetyltransferase, pyruvate oxidase, isocitrate lyase, fumarase, 2-oxoglutarate dehydrogenase, pyruvate kinase, malic enzyme, phosphoglucose isomerase, phosphoenolpyruvate carboxylase, phosphoenolpyruvate carboxykinase, pyruvate-formate lyase, succinic semialdehyde dehydrogenase, sugar-transporting phosphotransferase, ketohydroxyglutarate aldolase, homoserine-O-succinyl transferase, homoserine kinase, homoserine efflux transporter, diaminopimelate decarboxylase, and/or methylglyoxal synthase which is (are) decreased.

In a further aspect, the microorganism of the invention being Escherichia coli overexpresses at least one of the genes chosen among ppc, pck, aceA, galP, asd, thrA, metL, lysC all E. coli; pycA from L. lactis, and/or has at least one of the genes deleted chosen among ldhA, adhE, ackA, pta, poxB, focA, pfIB, sad, gabABC, sfcA, maeB, ppc, pykA, pykF, mgsA, sucAB, ptsl, ptsG, pgi, fumABC, aldA, lldD, icIR, metA, thrB, lysA, eda, rhtA, rhtB, rhtC.

The present invention also encompasses a method of production of 2,4-DHB comprising the steps of

-   -   culturing the modified microorganism of the invention in an         appropriate culture medium,     -   recovering 2,4-DHB from the culture medium. Said 2,4-DHB can be         further purified.

Product separation and purification is very important factor enormously affecting overall process efficiency and product costs. Methods for product recovery commonly comprise the steps cell separation, as well as product purification, concentration and drying, respectively.

Cell Separation

Ultrafiltration and centrifugation can be used to separate cells from the fermentation medium. Cell separation from fermentation media is often complicated by high medium viscosity. Therefore, we can add additives such as mineral acids or alkali salts, or heating of the culture broth to optimize cell separation.

Product Recovery

A variety of ion-exchange chromatographic methods can be applied for the separation of DHB either before or after biomass removal. They include the use of primary cation exchange resins that facilitate separation of products according to their isoelectric point. Typically, the resin is charged with the solution, and retained product is eluted separately following increase of pH (e.g. by adding ammonium hydroxide) in the eluent. Another possibility is the use of ion-exchange chromatography using fixed or simulated moving bed resins. Different chromatographic steps may have to be combined in order to attain adequate product purity. Those purification methods are more economical compared with a costly crystallization step, also providing additional advantages and flexibility regarding the form of final product.

Product Concentration and Drying

The purification process can also comprise a drying step which may involve any suitable drying means such as a spray granulator, spray dryer, drum dryer, rotary dryer, and tunnel dryer. Concentrated DHB solutions can be obtained by heating fermentation broths under reduced pressure by steam at 130° C. using a multipurpose concentrator or thin film evaporator.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1: Method of preparation of 2,4-DHB from homoserine comprising a two step pathway which employs a first step of conversion of the primary amino group of homoserine to a carbonyl group to obtain OHB, and a second step of reduction of the obtained OHB to 2,4-DHB.

FIG. 2: Specific activities of purified L. lactis lactate dehydrogenase mutated in position Q85. (A) specific activities on OHB, (B) specific activities on pyruvate, (C) Substrate specificity expressed as ratio of Vmax values on OHB and pyruvate. Values higher than 1 in graph C indicate preference for OHB (no saturation of enzymatic activity was obtained on either substrate for mutated enzymes between 0 and 50 mM OHB or pyruvate). Activities were measured at a substrate concentration of 20 mM.

FIG. 3: Specific activities of purified E. coli malate dehydrogenase mutated in position R81. (A) specific activities on OHB, (B) specific activities on oxaloacetate. Activities were measured at a substrate concentration of 20 mM OHB or 0.5 mM oxaloacetate.

The following non limiting examples illustrate the invention.

EXAMPLES Example 1 Demonstration of OHB Reductase Activity

Construction of plasmids containing wild-type genes coding for lactate dehydrogenase or malate dehydrogenase:

The genes coding for (L)-malate dehydrogenase in Escherichia coli, Ec-mdh (SEQ ID No. 1), (D)-lactate dehydrogenase in E. coli, Ec-ldhA (SEQ ID No. 3), (L)-lactate dehydrogenase of Lactococcus lactis, Ll-ldhA (SEQ ID No. 5), (L)-lactate dehydrogenase of Bacillus subtilis, Bs-ldh (SEQ ID No. 7), (L)-lactate dehydrogenase of Geobacillus stearothermophilus, Gs-ldh (SEQ ID No. 9), the two isoforms of the (L)-lactate dehydrogenase of Oryctalagus cuniculus, Oc-ldhA (SEQ ID No. 11 and SEQ ID No. 13), were amplified by PCR using the high-fidelity polymerase Phusion™ (Fermentas) and the primers listed in Table 1. Genomic DNAs of E. coli MG1655, L. Lactis IL1403, and B. subtilis strain 168 were used as the template. The genes Oc-IdhA, and Gs-ldh were codon-optimized for expression in E. coli and synthesized by MWG Eurofins. The primers introduced restriction sites (Table 1) upstream of the start codon and downstream of the stop codon, respectively, facilitating the ligation of the digested PCR products into the corresponding sites of the pET28a+ (Novagen) expression vector using T4 DNA ligase (Fermentas). Ligation products were transformed into E. coli DH5α cells (NEB). The resulting pET28-Ec-mdh, pET28-Ec-ldhA, pET28-Ll-ldhA, pET28-Bs-ldh, pET28-Gs-ldh, and pET28-Oc-ldhA plasmids were isolated and shown by DNA sequencing to contain the correct full-length sequence of the E. coli mdh, E. coli ldhA, L. lactis ldhA, B. subtilis ldh, G. stearothermophilus ldh, and O. cuniculus ldhA genes, respectively. The corresponding protein sequences are represented by SEQ ID No. 2, SEQ ID No. 4, SEQ ID No. 6, SEQ ID No. 8, SEQ ID No. 10, SEQ ID No. 12 and SEQ ID No. 14, respectively.

TABLE 1 Primer sequences and restriction sites used for amplification and cloning of candidate enzymes Forward and reverse Restriction Gene primer sequence 5′ - 3′ sites Ec-mdh TATAATCATATGAAAGTCGCAGTCCTC NdeI (SEQ ID No 15). BamHI TATAATGGATCCTTACTTATTAACGAACT C (SEQ ID No. 16) Ll-ldhA TATAATCATATGGCTGATAAACAACGTAA NdeI AAAA (SEQ ID No. 17) BamHI TATAATGGATCCTTAGTTTTTAACTGCAG AAGCAAA (SEQ ID No. 18) Bs_ldh TATAATGCTAGCATGATGAACAAACATGT NdeI AAATAAAGT (SEQ ID No. 19) BamHI TATAATGGATCCTTAGTTGACTTTTTGTT C (SEQ ID No. 20) Gs-ldh Gene was delivered by MWG NdeI Eurofins™ in pET28a vector BamHI Oc-ldhA TATAATGCTAGCATGGCGGCGTTGAAAGA NheI C (SEQ ID No. 21) EcoRI ATTATAGAATTCTTAAAATTGCAGTTCTT T (SEQ ID No. 22) Ll-panE TATAATCATATGAGAATTACAATTGCCGG NdeI (SEQ ID No. 23) BamHI TATAATGGATCCTTATTTTGCTTTTAATA ACTCTTCTTTGC (SEQ ID No. 24) Ec-ldhA TATAATCATATGAAACTCGCCGTTTATAG NdeI (SEQ ID No. 25) BamHI TATAATGGATCCTTAAACCAGTTCGTTCG G (SEQ ID No. 26)

Expression of enzymes: E. coli BL21 (DE3) star cells were transformed with the appropriate plasmids using standard genetic protocols (Sambrook, Fritsch, & Maniatis, 1989). Enzymes with an N-terminal hexa-His tag were expressed in 50 mL LB cultures that were inoculated from an overnight culture at OD₆₀₀ of 0.1 and grown to OD₆₀₀ of 0.6 before protein expression was induced by addition of 1 mM isopropyl β-D-1-thiogalactopyranoside (IPTG) to the culture medium. After 15 h of protein expression, cells were harvested by centrifugation at 4000 g at 4° C. for 10 min and discarding the supernatant. Cell pellets were stored at −20° C. until further analysis. Growth and protein expression were carried out at 25° C. Culture media contained 50 μg/mL kanamycin.

Purification of enzymes: Frozen cell pellets of expression cultures were resuspended in 0.5 mL of breakage buffer (50 mM Hepes, 300 mM NaCl, pH 7.5) and broken open by four successive rounds of sonication (sonication interval: 20 s, power output: 30%, sonicator: Bioblock Scientific, VibraCell™ 72437). Cell debris was removed by centrifuging the crude extracts for 15 min at 4° C. at 4000 g and retaining the clear supernatant. RNA and DNA were removed from the extracts by adding 15 mg/mL streptomycin sulfate (Sigma), centrifuging the samples at 13000 g for 10 min at 4° C. and retaining the supernatant. Clear protein extract was incubated for 1 h at 4° C. with 0.75 mL (bed volume) of Talon™ Cobalt affinity resin (Clontech). The suspension was centrifuged at 700 g in a table top centrifuge and supernatant was removed. The resin was washed with 10 bed volumes of wash buffer (50 mM Hepes, 300 mM NaCl, 15 mM Imidazole, pH 7.5) before proteins were eluted with 0.5 mL of elution buffer (50 mM Hepes, 300 mM NaCl, 250 mM Imidazole, pH 7.5). Purity of eluted enzymes was verified by SDS-PAGE analysis. Protein concentrations were estimated with the method of Bradford (Bradford (1976, Anal. Biochem. 72: 248-54). To stabilize the lactate dehydrogenase enzymes, the elution buffer was systematically exchanged by 100 mM phosphate buffer adjusted to pH 7. The protein sample was transferred to an Amicon™ Ultra centrifugal filter (cut-off 10 kDa), and centrifuged during 8 min at 4000 g at 4° C. to remove the buffer. The protein was diluted into phosphate buffer and the procedure was repeated 4 times.

Enzymatic assays: The reaction mixture contained 60 mM Hepes (pH 7), 50 mM potassium chloride, 5 mM MgCl₂, 0.25 mM NADH, (optionally 5 mM fructose-1,6-bisphosphate) (all products from Sigma), and appropriate amounts of purified malate or lactate dehydrogenase or cell extract. Reactions were started by adding appropriate amounts of 2-oxo-4-hydroxybutyrate (OHB), pyruvate, or oxaloacetate (OAA). Enzymatic assays were carried out at 37° C. in 96-well flat bottomed microtiter plates in a final volume of 250 μL. The reactions were followed by the characteristic absorption of NADH at 340 nm (ε_(NADH)=6.22 mM⁻¹ cm⁻¹) in a microplate reader (BioRad 680XR).

OHB was synthesized by incubating 125 mM homoserine with snake venom (L)-amino acid oxidase (1.25 U/mL, Sigma) and catalase (4400 U/mL, Sigma) in 100 mM Tris buffer at pH 7.8 for 90 min at 37° C. Subsequently, the reaction mixture was purified on an Amicon™ Ultra centrifugal filter with a cut-off of 10 kDa to eliminate the enzymes (method adapted from Wellner & Lichtenberg, 1971).

OHB was quantified by mixing 100 μL of the tested solution with 1 mL of a solution containing 1 M sodium arsenate and 1 M boric acid at pH 6.5. The mixture was incubated at room temperature for 30 min and the absorbance at 325 nm was used to quantify OHB. The relation between absorbance and concentration of the ketone was calibrated using pyruvate solutions of known concentrations (method adapted from (Wellner & Lichtenberg, 1971)). The typical OHB yield of the method was 90%.

Results: The kinetic parameters are listed in Table 2 for the tested enzymes on their natural substrates and OHB. Significant OHB reductase activity was found for all lactate dehydrogenases of different biological origin. Malate dehydrogenase, Mdh, of E. coli only had very minor activity on OHB. The branched chain 2-oxo-acid dehydrogenase, PanE, from L. lactis also had significant activity on OHB.

TABLE 2 Summary of kinetic parameters of selected candidate enzymes on their natural substrate and OHB Max. specific activity Substrate affinity, Km [μmol/(mg min)] [mM] Natural Natural Enzyme substrate^(a) OHB^(b) substrate^(a) OHB Ec-Mdh 95.6 0.01  0.04 ns Ll-Ldh 184 18 2.7 ns Gs-Ldh 87.7 66.8 1.2 1.3 Bs-Ldh 170 15.7 nd ns Ll-PanE nd 2.58 nd ns Oc-LdhA 68.3 6.5 1.5 13   Ec-LdhA 265 0.56 1.8 4.8 ^(a)Natural substrates for Mdh and Ldh are oxaloacetate and pyruvate, respectively ^(b)When enzymes could not be saturated, maximum specific activity refers to the activity estimated at 20 mM substrate concentration ns—not saturated nd—not determined

Example 2 Construction of Lactate Dehydrogenase Enzymes with Improved OHB Reductase Activity

Site-directed mutagenesis of the L. lactis ldhA gene was carried out using the pET28-Ll-ldhA plasmid as the template. Point mutations to change the amino acid sequence were introduced by PCR (Phusion 1U, HF buffer 20% (v/v), dNTPs 0.2 mM, direct and reverse primers 0.04 μM each, template plasmid 30-50 ng, water) using the oligonucleotide pairs listed in Table 3. The genes mutated by PCR contained a new restriction site listed in Table 3 (introduced using silent mutations) in addition to the functional mutation to facilitate identification of mutated clones. The PCR products were digested by Dpnl at 37° C. for 1 h to remove template DNA, and transformed into competent E. coli DH5α (NEB) cells. The mutated plasmids were identified by restriction site analysis and were verified to carry the desired mutations by DNA sequencing.

TABLE 3 Oligonucleotides used to mutate lactate dehydrogenase ldhA from L. lactis (nnk denotes a degenerated codon with k representing either thymine or cytosine) Primer sequences Restriction Protein Mutation 5′ - 3′ site Ll-LdhA Q85nnk GTCTTGACTTCTGGTG MluI CTCCANNKAAACCAGG TGAAACGCGTCTT (SEQ ID NO. 27) AAGACGCGTTTCACCT GGTTTMNNTGGAGCAC CAGAAGTCAAGAC  (SEQ ID NO. 28) Ll-LdhA I226V CGTGATGCTGCTTACT PvuI CGATCGTCGCTAAAAA AGGTG  (SEQ ID No. 99) CACCTTTTTTAGCGAC GATCGAGTAAGCAGCA TCACG  (SEQ ID No. 100) Mutant enzymes were expressed, purified and tested for OHB and pyruvate reductase activity as described in Example 1. The activity measurements for both substrates are summarized in FIG. 2. The results demonstrate that the replacement of Gln85 by preferably alanine, cysteine, asparagine, or methionine yields an increase of the enzyme's specificity for OHB, and/or an increase in maximum specific OHB reductase activity. The mutation Q85N in Ll-Ldh was combined with mutation I226V. It was demonstrated that this exchange had a major positive impact on substrate affinity for OHB.

TABLE 4 Summary of kinetic parameters of L. lactis lactate dehydrogenase A, Ll-LdhA, mutants on pyruvate and OHB Max. specific activity Km Mutant [μmol/(mg min)] [mM] Enzyme Seq ID Pyruvate OHB Pyruvate OHB Q85N SEQ ID No. 30 184 63.9 22.1 29.2 Q85NI226V SEQ ID No. 32 11.5 4.9 1.4 3.3

Example 3 Construction of Malate Dehydrogenase Enzymes with Improved OHB Reductase Activity

Site-directed mutagenesis of the mdh gene from E. coli was carried out as described in Example 2 using the primers listed in Table 5. Plasmid pET28-Ec-mdh was used as the template.

TABLE 5 Oligonucleotides used to mutate malate dehydrogenase mdh from E. coli. (nnk denotes a degenerated codon with k representing either thymine or cytosine) Primer sequences Restr. Protein Mutation 5′ - 3′ site Ec-Mdh R81nnk TTATCTCTGCAGGCGT Sma1 AGCGNNKAAACCCGGG ATGGATCGTTC (SEQ ID No. 33) GAACGATCCATCCCGG GTTTMNNCGCTACGCC TGCAGAGATAA (SEQ ID No. 34) Ec-Mdh R81AM85E TTATCTCTGCAGGCGT no AGCGGCTAAACCGGGT Sma1 GAGGATCGTTCCGACC TG (SEQ ID NO. 35) CAGGTCGGAACGATCC TCACCCGGTTTAGCCG CTACGCCTGCAGAGAT AA (SEQ ID NO. 36) Ec-Mdh R81AM85Q TTATCTCTGCAGGCGT no AGCGGCTAAACCGGGT Sma1 CAGGATCGTTCCGACC TG (SEQ ID NO. 37) CAGGTCGGAACGATCC TGACCCGGTTTAGCCG CTACGCCTGCAGAGAT AA  (SEQ ID NO. 38) Ec-Mdh I12V GTCGCAGTCCTCGGCG Nar1 CCGCTGGCGGTGTCGG CCAGGCGCTTGCAC  (SEQ ID NO. 39 GTGCAAGCGCCTGGCC GACACCGCCAGCGGCG CCGAGGACTGCGAC  (SEQ ID NO. 40) Ec-Mdh G179D CCG GTT ATT GGC Eae1 GGC CAC TCT GAT GTT ACC ATT CTG CCG CTG CTG (SEQ ID NO. 41) CAGCAGCGGCAGAATG GTAACATCAGAGTGGC CGCCAATAACCGG  (SEQ ID NO. 42) Ec-Mdh R81AD86S GGCGTAGCGGCTAAAC no CGGGTATGTCTCGTTC Sma1 CGACCTG (SEQ ID NO. 43) CAGGTCGGAACGAGAC ATACCCGGTTTAGCCG CTACGCC (SEQ ID NO. 44) Mutant enzymes were expressed, purified and tested for OHB and oxaloacetate reductase activity as described in Example 1. The activity measurements on OHB and oxaloacetate are summarized in FIG. 3. The results demonstrate that replacement of Arg81 by alanine, cysteine, glycine, histidine, isoleucine, leucine, methionine, asparagine, glutamine, serine, threonine, or valine confer significant OHB reductase activity, and concomitant decrease of oxaloacetate reductase activity. The mutation R81A in Ec-Mdh was combined with additional changes in the protein sequence. The results are listed in Table 6. It was demonstrated that the introduction of mutations M85Q, M85E, I12V, D86S or G179D result in an increased activity on OHB.

TABLE 6 Summary of kinetic parameters of E. coli malate dehydrogenase mutants on oxaloacetate (OAA) and OHB Max. specific activity Km Mutant [μmol/(mg min)] [mM] Enzyme Seq ID OAA^(a) OHB^(b) OAA OHB Wild-type SEQ ID No. 95 0.01 0.04 ns 2 R81A SEQ ID No. 1.16 1.8 ns ns 102 R81A SEQ ID No. 0.5 4.99 ns ns M85Q 104 R81A SEQ ID No. 1 3 ns ns M85E 106 R81A SEQ ID No. 1.84 18.9 ns 15 M85Q I12V 108 R81A SEQ ID No. 2.2 12.54 ns ns M85E I12V 110 R81A SEQ ID No. 0.37 4.16 ns ns G179D 112 R81A D86S SEQ ID No. 0.67 14.6 ns ns 1114 R81A 112V SEQ ID No. 0.5 4.9 ns ns 115 R81A SEQ ID No. 0.54 19 ns ns G179D 118 D86S ^(a)activity was measured at 0.5 mM oxaloacetate ^(b)activity was measured at 20 mM OHB ns—not saturated at concentrations of up to 50 mM of OHB and 0.5 mM of oxaloacetate

Example 4 Demonstration of Homoserine Transaminase Activity for Selected Transaminases

The genes coding for different transaminases in E. coli, S. cerevisiae, and L. lactis were amplified by PCR using the high-fidelity polymerase Phusion™ (Finnzymes) and the primers listed in Table 7. Genomic DNA of E. coli MG1655, S. cerevisiae BY4741, and L. lactis IL1403 were used as the templates. The primers introduced restriction sites (Table 7) upstream of the start codon and downstream of the stop codon, respectively, facilitating the ligation of the digested PCR products into the corresponding sites of the pET28a+(Novagen) expression vector using T4 DNA ligase (Biolabs). Ligation products were transformed into E. coli DH5a cells. The resulting plasmids were isolated and shown by DNA sequencing to contain the correct full-length sequence of the corresponding genes. The references to the corresponding protein sequences are listed in Table 7.

TABLE 7 Primer sequences and restriction sites used for amplification and cloning of candidate enzymes (Abbreviations used for source organism: Ec - E. coli, Sc - S. cerevisiae, Ll - L. lactis). All the genes were cloned into pET28a+ (Novagen), adding an N-terminal Hexa-HisTag. Forward and reverse  Gene Protein Restriction Gene primer sequences 5′ - 3′ sequence sequence sites Ec-ilvE tataatgctagcatgaccacgaagaaagctgattaca SEQ ID SEQ ID NheI (SEQ ID No. 47) No. 59 No. 60 BamHI tataatggatccttattgattaacttgatctaacc (SEQ ID No. 48) Ec-tyrB Tataatgctagcgtgtttcaaaaagttgacg SEQ ID SEQ ID NheI (SEQ ID No. 49) No. 61 No. 62 BamHI Tataatggatccttacatcaccgcagcaaac (SEQ ID No. 50) Ec-aspC Tataatgctagcatgtttgagaacattaccgc SEQ ID SEQ ID NheI (SEQ ID No. 51) No. 63 No. 64 BamHI Tataatggatccttacagcactgccacaatcg (SEQ ID No. 52) Ll-araT Tataatgctagcatggatttattaaaaaaatttaaccctaa SEQ ID SEQ ID NheI (SEQ ID No. 53) No. 65 No. 66 BamHI Tataatggatcctcagccacgttttttagtcacataa (SEQ ID No. 54) Ll-bcaT Tataatgctagcatggcaattaatttagactg SEQ ID SEQ ID NheI (SEQ ID No. 55) No. 67 No. 68 BamHI Tataatggatccttaatcaactttaactatcc (SEQ ID No. 56) Sc-ARO8 Tataatcatatgatcatgactttacctgaatcaaaaga SEQ ID SEQ ID NheI (SEQ ID No. 57) No. 69 No. 70 BamHI Tataatggatccctatttggaaataccaaattcttcg (SEQ ID No. 58) Enzymes were expressed and purified as described in Example 1, and tested for homoserine transaminase activity under the conditions described below.

Enzymatic assays: Transaminase activity of several candidate aminotransferases was quantified with 2-oxoglutarate as the amino group acceptor. Transaminase reactions were carried out using homoserine and the preferred amino acid of the enzymes. The reactions were followed by the amino acid-dependent oxidation of NADH in the coupled dehydrogenase reaction.

Transaminase Assays (Reaction Scheme)

Transaminase: Amino acid+2-oxoglutarate->2-oxo-acid+glutamate Dehydrogenase: 2-oxo-acid+NADH->2-hydroxy-acid+NAD⁺

The reaction mixture contained 60 mM Hepes (pH 7), 50 mM potassium chloride, 5 mM MgCl₂, 4 mM 2-oxoglutarate, 0.1 mM pyridoxal-5′-phosphate (PLP), 0.25 mM NADH, (optionally 5 mM fructose-1,6-bisphosphate) (all products from Sigma), 4 Units/mL of auxiliary 2-hydroxyacid dehydrogenase, and appropriate amounts of purified aminotransferase or cell extract. The auxiliary dehydrogenase enzyme was purified PanE from L. lactis in case of the amino acids phenylalanine and leucine (Chambellon, Rijnen, Lorquet, Gitton, van HylckamaVlieg, Wouters, &Yvon, 2009), malate dehydrogenase (Sigma) in case of aspartate, and rabbit muscle (L)-lactate dehydrogenase (Sigma) when homoserine was used as the starting substrate. Reactions were started by adding 50 mM of the amino acid.

Enzymatic assays were carried out at 37° C. in 96-well flat bottomed microtiter plates in a final volume of 250 μL. The reactions were followed by the characteristic absorption of NAD(P)H at 340 nm (ε_(NADPH)=6.22 mM⁻¹ cm⁻¹) in a microplate reader (BioRad 680XR).

Results: The kinetic parameters of different aminotransferases are listed in Table 8. Significant homoserine transaminase activity was found for the listed transaminase enzymes.

TABLE 8 Transaminase activities of tested candidate enzymes on homoserine and their preferred amino acid substrate (Abbreviations used for source organism: Ec—E. coli, Sc—S. cerevisiae, Ll—L. lactis). Max. specific activity on different substrates [μmol/(min mg_(prot))] Enzyme Homoserine* Preferred amino acid Ec-IlvE 0.077 10.3^((L)) Ec-TyrB 0.057 9.03^((P)) Ec-AspC 0.082 74.031^((A)) Ll-AraT 0.109 11.72^((P)) Ll-BcaT 0.028 30.39^((L)) Sc-ARO8 0.076 20.5^((P)) *activity measured at 50 mM homoserine,

Example 5 Construction of Plasmids for Overexpression of the Homoserine Pathway Enzymes

Construction of the Plasmids pTAC-op-HMS1 and pACT3-op-HMS1

The plasmid pET28-LYSCwt was constructed by amplifying the lysC gene by PCR using high fidelity polymerase Phusion™ (Finnzymes) and the direct and reverse primers ^(5′)CACGAGGTACATATGTCTGAAATTGTTGTCTCC^(3′) (SEQ ID No. 71) and ^(5′)CTTCCAGGGGATCCAGTATTTACTCAAAC^(3′) (SEQ ID No. 72) that introduced a NdeI and BamHI restriction sites upstream of the start codon and downstream of the stop codon, respectively. Genomic DNA from E. coli MG1655 was used as the template. The PCR product was digested with NdeI and BamHI, ligated into the corresponding sites of the pET28a (Novagen) expression vector using T4 DNA ligase (Biolabs), and transformed into E. coli DH5α cells. The resulting pET28-LYSCwt plasmid was isolated and shown by DNA sequencing to contain the full-length lysC gene having the correct sequence (SEQ ID No. 73).

Site-directed mutagenesis of lysC to alleviate inhibition by lysine was carried out using the pET28-LYSCwt plasmid as the template. A point mutation to change the amino acid sequence in position 250 from glutamate to lysine (E250K, SEQ ID No. 36) was introduced by PCR (Phusion 1U, HF buffer 20% (v/v), dNTPs 0.2 mM, direct and reverse primers 0.04 μM each, template plasmid 50 ng, water) using the oligonucleotides ^(5′)GCGTTTGCCGAAGCGGCAAAGATGGCCACTTTTG^(3′) (SEQ ID No. 74) and ^(5′)CAAAAGTGGCCATCTTTGCCGCTTCGGCAAACGC^(3′) (SEQ ID No. 75). The PCR product (SEQ ID No. 35) was digested by DpnI at 37° C. for 1 h to remove template DNA, and transformed into competent E. coli DH5α (NEB) cells. The mutated plasmid pET28-LYSC* was identified by restriction site analysis and verified to carry the desired mutations by DNA sequencing.

The plasmid pET28-ASDwt was constructed by amplifying the asd gene of E. coli by PCR using high fidelity polymerase Phusion™ (Finnzymes) and the direct and reverse primers ^(5′)TATAATGCTAGCATGAAAAATGTTGGTTTTATCGG^(3′) (SEQ ID No. 76) and ^(5′)TATAATGGA-TCCTTACGCCAGTTGACGAAGC^(3′) (SEQ ID No. 77) that introduced a NheI and BamHI restriction site upstream of the start codon and downstream of the stop codon, respectively. Genomic DNA from E. coli DH5α was used as the template. The PCR product was digested with NheI and BamHI, ligated into the corresponding sites of the pET28a (Novagen) expression vector using T4 DNA ligase (Biolabs), and transformed into E. coli DH5α cells. The resulting pET28-ASDwt plasmid was isolated and shown by DNA sequencing to contain the full-length asd gene having the correct sequence (SEQ ID No. 98).

The plasmid pET28-HOM6wt was constructed by amplifying the HOM6 gene of S. cerevisiae by PCR using high fidelity polymerase Phusion™ (Finnzymes) and the direct and reverse primers ^(5′)TATAATCATATGAGCACTAAAGTTGTTAATG^(3′) (SEQ ID No. 78) and ^(5′)TATAATGGATC-CCTAAAGTCTTTGAGCAATC^(3′) (SEQ ID No. 79) that introduced a NdeI and BamHI restriction site upstream of the start codon and downstream of the stop codon, respectively. Genomic DNA from S. cerevisiae BY4741 was used as the template. The PCR product was digested with NdeI and BamHI, ligated into the corresponding sites of the pET28a (Novagen) expression vector using T4 ligase (Biolabs), and transformed into E. coli DH5α cells. The resulting pET28-HOM6wt plasmid was isolated and shown by DNA sequencing to contain the full-length HOM6 gene having the correct sequence (SEQ ID No. 97).

The plasmid pET28-LYSC* was used as the backbone for the construction of the pTAC-op-HMS plasmid that enabled the expression of lysine-insensitive aspartate kinase, aspartate semialdehyde dehydrogenase, and homoserine dehydrogenase from an inducible tac promoter.

The asd gene was obtained by PCR from pET28-asdwt. The whole coding region and part of the upstream region comprising the pET28 ribosome binding site (rbs) and the in-frame N-terminal His-Tag were amplified by PCR using high fidelity polymerase Phusion™ (Finnzymes) and the direct and reverse primers ^(5′)TATAAGGATCCGTTTAACTTTAAGAAGGAGATATACCATGGG^(3′) (SEQ ID No. 80) and ⁵TATAAGAATTCTTACGCCAGTTGACGAAG^(3′) (SEQ ID No. 81) that introduced a BamHI and EcoRI restriction site upstream of the rbs and downstream of the stop codon, respectively. The PCR product was digested with BamHI and EcoRI, ligated into the corresponding sites of pET28-LYSC*, using T4 DNA ligase (Biolabs), and transformed into E. coli DH5α cells. The resulting pET28-LYSC*-ASD plasmid was isolated and shown by DNA sequencing to have the correct sequence.

The HOM6 gene was obtained by PCR from pET28-HOM6wt. The whole coding region and part of the upstream region comprising the pET28 ribosome binding site and the in-frame N-terminal His-Tag were amplified by PCR using high fidelity polymerase Phusion™ (Finnzymes), the direct primer ^(5′)TATAAGCGGCCGCGTTTAACTTTAAGAAGGAGATAT^(3′) (SEQ ID No. 82), and the reverse primer ^(5′)TATAAACTCGAGCCTAAAGTCTTTGAGCAAT^(3′) (SEQ ID No. 83) that introduced a Notl and a PspXI restriction site upstream of the rbs and downstream of the stop codon, respectively. The PCR product was digested with NotI and PspXI, ligated into the corresponding sites of pET28-LYSC*-ASD, using T4 DNA ligase (Biolabs), and transformed into E. coli DH5α cells. The resulting pET28-op-HMS1 plasmid was isolated and shown by DNA sequencing to have the correct sequence.

The 5′ upstream promoter region simultaneously regulating the expression of the three genes (i.e. the T7 promoter in pET28a+) can be replaced with any other promoter, inducible or constitutive, by digesting the plasmids with SphI and XbaI and cloning another promoter region with suitable restriction sites.

In the present non-exclusive example, the T7 promoter of the pET28a+backbone was replaced by the artificial IPTG-inducible tac promoter (de Boer et al., 1983). The tac promoter was obtained from plasmid pEXT20 (Dykxhoorn et al., 1996) by digesting this plasmid with SphI and XbaI. The DNA fragment containing the promoter was purified and cloned into SphI and XbaI digested pET28-op-HMS1 obtaining pTAC-op-HMS1. The resulting pTAC-op-HMS plasmid was isolated and shown by DNA sequencing to have the correct sequence.

The operon containing the coding sequences of lysC*, asd, and HOM6 was PCR amplified from the plasmid pTAC-op-HMS1 using the primers 5′-TATAAAGATCTTAGAAATAATTTTGTTTA-3′ (SEQ ID No. 84) and 5′-TATAATCTAGACTAAAGTCTTTGAGCAAT-3′ (SEQ ID No. 85) which introduced a BgIII and a XbaI restriction site at the 5′ and the 3′ end, respectively, of the PCR fragment. The fragment was purified, digested with BgIII and XbaI and cloned into the corresponding sites of pACT3 (Dykxhoorn et al., 1996) to obtain the vector pACT3-op-HMS1. The resulting pACT3-op-HMS1 plasmid was isolated and shown by DNA sequencing to have the correct sequence.

Construction of the Plasmids pEXT20-op-HMS2 and pACT3-op-HMS2

The plasmid pET28-thrAwt was constructed by amplifying the E. coli thrA gene encoding bifunctional enzyme aspartate kinase/homoserine dehydrogenase I by PCR using high fidelity polymerase Phusion™ (Finnzymes) and the direct and reverse primers 5′-TATAATCATATGCGAGTGTTGAAGTTCG-3′ (SEQ ID No. 86) and 5′-TATAATGGATCCTCAGACTCCTAACTTCCA-3′ (SEQ ID No. 87) that introduced a NdeI and BamHI restriction sites upstream of the start codon and downstream of the stop codon, respectively. Genomic DNA from E. coli MG1655 was used as the template. The PCR product was digested with NdeI and BamHI, ligated into the corresponding sites of the pET28a+ (Novagen) expression vector using T4 DNA ligase (Biolabs), and transformed into NEB 5-alpha competent E. coli cells (NEB). The resulting pET28-thrAwt plasmid was isolated and shown by DNA sequencing to contain the full-length thrA gene having the correct sequence (SEQ ID No.88). The corresponding protein is represented by SEQ ID No.89.

An aspartate kinase/homoserine dehydrogenase with strongly decreased sensitivity for inhibition by threonine was constructed by site directed mutagenesis, replacing serine in position 345 with phenylalanine (S345F). Site-directed mutagenesis was carried out using the direct and reverse primers 5′-TGTCTCGAGCCCGTATTTTCGTGGTGCTG-3′ (SEQ ID No. 90) and 5′-CAGCACCACGAAAATACGGGCTCGAGACA-3′ (SEQ ID No.91) and the pET28-thrAwt plasmid as the template. A single point mutation to change the amino acid sequence was introduced by PCR (Phusion 1U, HF buffer 20% (v/v), dNTPs 0.2 mM, direct and reverse primers 0.04 μM each, template plasmid 30-50 ng, water). Plasmids created by PCR contained a new restriction site for XhoI (underlined) introduced by silent mutation in addition to the functional mutation to facilitate identification of mutated clones. The PCR products were digested by DpnI at 37° C. for 1 h to remove template DNA, and transformed into DH5α competent E. coli cells (NEB). The mutated plasmid pET_Ec_thrA_S345F was identified by restriction site analysis and verified to carry the desired mutation by DNA sequencing.

The thrAS345F coding region of the bifunctional E. coli aspartate kinase/homoserine dehydrogenase was obtained by PCR using the plasmid pET_Ec_thrA_S345F as the template (SEQ ID No. 92). The whole coding region was amplified by PCR using high fidelity polymerase Phusion™ (Finnzymes) and the direct and reverse primers 5′-TATAATGAGCTCGTTTAACTTTAAGAAGGAGATATACCATGCGAGTGTTGA AGTTCGGCG-3′ (SEQ ID No. 93) and 5′-TATAATCCCGGGTCAGACTCCTAACTTCCA-3′ (SEQ ID No. 94) that introduced a SacI and XmaI restriction site (underlined) upstream of the start codon and downstream of the stop codon, respectively. The direct primer includes the ribosome binding site (bold face) sequence of pET28. The PCR product was digested with SacI and XmaI, ligated into the corresponding sites of either pEXT20 or pACT3 (Dykxhoorn, St Pierre, & Linn, 1996), using T4 DNA ligase (Biolabs), and transformed into E. coli DH5α cells. The resulting pEXT20-op-HMS2_step1 and pACT3-op-HMS2_step1 plasmids were isolated and shown by DNA sequencing to have the correct sequence.

Escherichia coli aspartate semialdehyde dehydrogenase asd was amplified by PCR using high fidelity polymerase Phusion™ (Finnzymes) and the direct and reverse primers 5′-TATAATCCCGGGGTTTAACTTTAAGAAGGAGATATACCATGAAAAATGTTG GTTTTATCGGC-3′ (SEQ ID No. 95) and 5′-TATAATGGATCCTTACGCCAGTTGACGAAG-3′ (SEQ ID No. 96) that introduced a XmaI and BamHI restriction site upstream of the start codon and downstream of the stop codon, respectively(SEQ ID No. 98). The direct primer includes the ribosome binding site sequence of pET28. Genomic DNA of E coli MG1655 was used as the template. The PCR product was digested with XmaI and BamHI, ligated into the corresponding sites of pEXT20-op-HMS2_step1 and pACT3-op-HMS2_step1, directly downstream the E. coli thrA gene, using T4 DNA ligase (Biolabs), and transformed into E. coli DH5α cells. The resulting pEXT20-op-HMS2 and pACT3-op-HMS2 plasmids were isolated and shown by DNA sequencing to have the correct sequence.

Example 6 Construction of Plasmids for Overexpression of Phosphoenolpyruvate (PEP) Carboxykinase, PEP Carboxylase, Pyruvate Kinase, Pyruvate Carboxylase, Isocitrate Lyase Enzymes and the Galactose Symporter Permease

The plasmid pACT3-pck harbouring the PEP carboxykinase encoding pck gene of E. coli was constructed by amplifying the pck coding sequence using genomic DNA from E. coli MG1655 as the template and the forward and reverse primers, respectively,

^(5′)TATAATCCCGGGATGCGCGTTAACAATGGTTTGACC^(3′) (SEQ ID No. 119) and ^(5′)TATAATTCTAGATTACAGTTTCGGACCAGCCG^(3′) (SEQ ID No. 120). The DNA fragment was digested with XmaI and XbaI, ligated into the corresponding sites of the pACT3 expression vector (Dykxhoorn et al., 1996) using T4 DNA ligase (Biolabs), and transformed into E. coli DH5α cells. The transformants were selected on solid LB medium containing chloramphenicol (25 μg/mL). The resulting plasmid was isolated and correct insertion of the pck gene was verified by sequencing. Plasmids pACT3-aceA, pACT3-ppc, pACT3-galP, pACT3-pck and pACT3-pycA harbouring, respectively, aceA, ppc, galP, or pck (all E. coli) or pycA from Lactococcus lactis were constructed analogously using the primers listed in Table 9.

TABLE 9 Primers used for construction of plasmids for gene overexpression. Restriction sites used for cloning into pACT3 are underlined Gene Primer Linker Sequence Ec_pck Ec_pck_clon_for XmaI tataatcccgggatgc gcgttaacaatggttt gacc (SEQ ID No. 121) Ec_pck_clon_rev XbaI tataattctagattac agtttcggaccagccg (SEQ ID No. 122) Ec_ppc Ec_ppc_clon_for XmaI tataatcccgggatga acgaacaatattcc (SEQ ID No. 123) Ec_ppc_clon_rev XbaI tataattctagattag ccggtattacgcat (SEQ ID No. 124) Ec_aceA Ec_aceA_clon_for XmaI tataatcccgggatga aaacccgtacacaaca aatt (SEQ ID No. 125) Ec_aceA_clon_rev XbaI tataattctagattag aactgcgattcttcag (SEQ ID No. 126) Ll_pycA Ll_pycA_clon_for XmaI tataatcccgggatga aaaaactactcgtcgc caat (SEQ ID No. 127) Ll_pycA_clon_rev XbaI tataattctagattaa ttaatttcgattaaca (SEQ ID No. 128) Ec_galP Ec_galP_clon_for XmaI tataatcccgggatgc ctgacgctaaaaaaca ggggcggt (SEQ ID No. 129) Ec_galP_clon_rev XbaI tataattctagattaa tcgtgagcgcctattt c (SEQ ID No. 130)

Example 7 Construction of the Plasmid for Overexpression of the Homoserine Transaminase and the OHB Reductase

The coding sequence of the branched chain amino transferase, IlvE, from E. coli was PCR amplified using the forward and reverse primers 5′-ACAATTTCACACAGGAAACAGAATTCGAGCTCGGTACCGTTTAACTTTAAG AAGGAGATATACCATGACCACGAAGAAAGCTGATTAC-3′ (SEQ ID No. 131) and 5′-GGATAACTTTTTTACGTTGTTTATCAGCCATGGTATATCTCCTTCTTAAAGT TAAACGGATCCTTATTGATTAACTTG-3′ (SEQ ID No. 132), respectively, and plasmid pET28-Ec-ilvE (Example 4) as the template. The coding sequence of lactate dehydrogenase, LdhA, from L. lactis was PCR amplified using the forward and reverse primers 5′-TAATATGGATCCGTTTAACTTTAAGAAGGAGATATACCATGGCTGATAAAC AACGTAAAAAAGTTATCC-3′ (SEQ ID No. 133) and 5′-CAATGCGGAATATTGTTCGTTCATGGTATATCTCCTTCTTAAAGTTAAACTC TAGATTAGTTTTTAACTGCAGAAGCAAATTC-3′ (SEQ ID No. 134), respectively, and plasmid pET28-Ll-ldhA (Example 1) as the template. The amplified PCR fragments were fused in an overlap extension PCR by adding 150 ng of each fragment to 50 μL of the reaction mix and running a PCR using primers

5′-ACAATTTCACACAGGAAACAGAATTCGAGCTCGGTACCGTTTAACTTTAAG AAGGAGATATACCATGACCACGAAGAAAGCTGATTAC-3′ (SEQ ID No. 135) and 5′-CAATGCGGAATATTGTTCGTTCATGGTATATCTCCTTCTTAAAGTTAAACTC TAGATTAGTTTTTAACTGCAGAAGCAAATTC-3′ (SEQ ID No. 136). The resulting PCR fragment was purified, digested with KpnI and XbaI, and ligated into the corresponding sites of pEXT20 (Dykxhoorn, St Pierre, & Linn, 1996) using T4 DNA ligase (Fermentas). The ligation product was transformed into E. coli DH5α. The resulting plasmid pEXT20-DHB was isolated and shown by DNA sequencing to contain the correct full-length coding sequences of Ec-ilvE and Ll-ldhA. The plasmid was then transformed into E. coli MG1655-derived mutant strains and tested regarding DHB production.

Example 8 Construction of Optimized Strains for DHB Production

Several genes were disrupted in E. coli strain MG1655 in order to optimise carbon flux repartitioning and cofactor supply for DHB production. Gene deletions were carried out using phage transduction method, or the lambda red recombinase method according to Datsenko et al. (Datsenko & Wanner, 2000).

Protocol for Introduction of Gene Deletions Using the Phage Transduction Method:

Strains carrying the desired single deletions were obtained from the Keio collection (Baba et al., 2006). Phage lysates of single deletion mutants were prepared by inoculating 10 mL of LB medium containing 50 μg/mL kanamycin, 2 g/L glucose, and 5 mM CaCl₂ with 100 μL of overnight precultures. Following an incubation of 1 h at 37° C., 200 μL of phage lysate prepared from the wild-type MG1655 strain were added, and cultures were incubated for another 2-3 h until cell lysis had completed. After addition of 200 μL chloroform, cell preparations were first vigorously vortexted and then centrifuged for 10 min at 4500×g. The clear lysate was recovered and stored at 4° C.

The receptor strain was prepared for phage transduction by an overnight cultivation at 37° C. in LB medium. A volume of 1.5 mL of the preculture was centrifuged at 1500×g for 10 min. The supernatant was discarded and the cell pellet was resuspended in 600 μl of a solution containing 10 mM MgSO₄ and 5 mM CaCl₂. The transduction was carried out by mixing 100 μL of the solution containing the receptor strain with 100 μL of lysate and incubating this mixture at 30° C. for 30 min. Thereafter, 100 μL of a 1M sodium citrate solution were added followed by vigorous vortexing. After addition of 1 mL LB medium, the cell suspension was incubated at 37° C. for 1 h before spreading the cells on LB agar dishes containing 50 μg/mL kanamycin. Clones able to grow in presence of the antibiotic were confirmed by colony PCR to contain the desired deletion using the primers listed in Table 11. After the introduction of each gene deletion, the antibiotic marker was removed as described above following the method of (Cherepanov & Wackernagel, 1995). The deletions ΔldhA, ΔadhE, ΔmetA, ΔthrB, ΔrhtB, and ΔlldD were successively introduced by the described method.

Protocol for Introduction of Gene Deletions Using the Lambda-Red Recombinase Method:

The deletion cassettes were prepared by PCR using high fidelity polymerase Phusion™ (Finnzymes), and the FRT-flanked kanamycin resistance gene (kan) of plasmid pKD4 as the template (Datsenko & Wanner, 2000). Sense primers contained sequences corresponding to the 5′ end of each targeted gene (underlined) followed by 20 bp corresponding to the FRT-kan-FRT cassette of pKD4. Anti-sense primers contained sequences corresponding to the 3′ end region of each targeted gene (underlined) followed by 20 bp corresponding to the cassette. The primers are described in Table 10. PCR products were digested with DpnI and purified prior to transformation.

E. coli MG1655 strain was rendered electro-competent by growing the cells to an OD₆₀₀ of 0.6 in LB liquid medium at 37° C., concentrating the cells 100-fold, and washing them twice with ice-cold 10% glycerol. The cells were transformed with plasmid pKD46 (Datsenko & Wanner, 2000) by electroporation (2.5 kV, 200 Ω, 25 μF, in 2 mm gap cuvettes). Transformants were selected at 30° C. on ampicillin (100 μg/mL) LB solid medium.

Disruption cassettes were transformed into electro-competent E. coli strains harbouring the lambda Red recombinase-expressing plasmid pKD46. The cells were grown at 30° C. in liquid SOB medium containing ampicillin (100 μg/mL). The lambda red recombinase system was induced by adding 10 mM arabinose when OD₆₀₀ of the cultures reached 0.1. Cells were further grown to an OD₆₀₀ of 0.6 before they were harvested by centrifugation, washed twice with ice-cold 10% glycerol, and transformed with the disruption cassette by electroporation. After an overnight phenotypic expression at 30° C. in LB liquid medium, cells were plated on solid LB medium containing 25 μg/mL kanamycin. Transformants were selected after cultivation at 30° C.

The gene replacement was verified by colony PCR using Crimson Taq polymerase (NEB). A first reaction was carried out with the flanking locus-specific primers (see Table 11) to verify simultaneous loss of the parental fragment and gain of the new mutant specific fragment. Two additional reactions were done by using one locus-specific primer together with one of the corresponding primers k1 rev, or k2 for (see Table 11) that align within the FRT-kanamycin resistance cassette (sense locus primer/k1 rev and k2for/reverse locus primer).

The resistance gene (FRT-kan-FRT) was subsequently excised from the chromosome using the FLP recombinase-harbouring plasmid pCP20 (Cherepanov & Wackernagel, 1995) leaving a scar region containing one FRT site. pCP20 is an ampicillin and CmR plasmid that shows temperature-sensitive replication and thermal induction of FLP recombinase synthesis. Kanamycin resistant mutants were transformed with pCP20, and ampicillin-resistant transformants were selected at 30° C. Transformants were then grown on solid LB medium at 37° C. and tested for loss of all antibiotic resistances. Excision of the FRT-kanamycin cassette was analysed by colony PCR using crimson taq polymerase and the flanking locus-specific primers (Table 11). Multiple deletions were obtained by repeating the above described steps.

TABLE 10 Primers used for gene disruptions. Sequences homologous to target genes are underlined Gene Primer Sequence ldhA Δ_ldhA_for gaaggttgcgcctacactaagcatagttgttgatgagtgtaggctggagctgcttc (SEQ ID No. 137) Δ_ldhA_rev ttaaaccagttcgttcgggcaggtttcgcctttttcatgggaattagccatggtcc SEQ ID No. 138) adhE Δ_adhE_for atggctgttactaatgtcgctgaacttaacgcactcgtagagcgtgtgtaggctggagctgcttc (SEQ ID No. 139) Δ_adhE_rev ttaagcggattttttcgcttttttctcagctttagccggagcagccatatgaatatcctccttag (SEQ ID No. 140) ackA Δ_ackA_for atgtcgagtaagttagtactggttctgaactgcggtagttcttcagtgtaggctggagctgcttc (SEQ ID No. 141 Δ_ackA_rev tcaggcagtcaggcggctcgcgtcttgcgcgataaccagttcttccatatgaatatcctccttag (SEQ ID No. 142) focA- Δ_focA-pflB_for ttactccgtatttgcataaaaaccatgcgagttacgggcctataagtgtaggctggagctgcttc pflB (SEQ ID No. 143) Δ_focA-pflB_rev atagattgagtgaaggtacgagtaataacgtcctgctgctgttctcatatgaatatcctccttag (SEQ ID No. 144) pta Δ_pta_for gtgtcccgtattattatgctgatccctaccggaaccagcgtcggtgtgtaggctggagctgcttc (SEQ ID No. 145) Δ_pta_rev ttactgctgctgtgcagactgaatcgcagtcagcgcgatggtgtacatatgaatatcctccttag (SEQ ID No. 146) poxB Δ_poxB_for atgaaacaaacggttgcagcttatatcgccaaaacactcgaatcggtgtaggctggagctgcttc (SEQ ID No. 147) Δ_poxB_rev ttaccttagccagtttgttttcgccagttcgatcacttcatcacccatatgaatatcctccttag (SEQ ID No. 148) sad Δ_sad_for atgaccattactccggcaactcatgcaatttcgataaatcctgccgtgtaggctggagctgcttc (SEQ ID No. 149) Δ_sad_rev tcagatccggtctttccacaccgtctggatattacagaattcgtgcatatgaatatcctccttag (SEQ ID No. 150) gabD Δ_gabD_for atgaaacttaacgacagtaacttattccgccagcaggcgttgattgtgtaggctggagctgcttc (SEQ ID No. 151) Δ_gabD_rev ttaaagaccgatgcacatatatttgatttctaagtaatcttcgatcatatgaatatcctccttag (SEQ ID No. 152) gadA Δ_gadA_for atggaccagaagctgttaacggatttccgctcagaactactcgatgtgtaggctggagctgcttc (SEQ ID No. 153) Δ_gadA_rev tcaggtgtgtttaaagctgttctgctgggcaataccctgcagtttcatatgaatatcctccttag (SEQ ID No. 154) gadB Δ_gadB_for atggataagaagcaagtaacggatttaaggtcggaactactcgatgtgtaggctggagctgcttc (SEQ ID No. 155) Δ_gadB_rev tcaggtatgtttaaagctgttctgttgggcaataccctgcagtttcatatgaatatcctccttag (SEQ ID No. 156) gadC Δ_gadC_for atggctacatcagtacagacaggtaaagctaagcagctcacattagtgtaggctggagctgcttc (SEQ ID No. 157) Δ_gadC_rev ttagtgtttcttgtcattcatcacaatatagtgtggtgaacgtgccatatgaatatcctccttag (SEQ ID No. 158) sfcA Δ_sfcA_for atggaaccaaaaacaaaaaaacagcgttcgctttatatcccttacgtgtaggctggagctgcttc (SEQ ID No. 159) Δ_sfcA_rev ttagatggaggtacggcggtagtcgcggtattcggcttgccagaacatatgaatatcctccttag (SEQ ID No. 160) maeB Δ_maeB_for atggatgaccagttaaaacaaagtgcacttgatttccatgaatttgtgtaggctggagctgcttc (SEQ ID No. 161) Δ_maeB_rev ttacagcggttgggtttgcgcttctaccacggccagcgccaccatcatatgaatatcctccttag (SEQ ID No. 162) ppc Δ_ppc_for atgaacgaacaatattccgcattgcgtagtaatgtcagtatgctcgtgtaggctggagctgcttc (SEQ ID No. 163) Δ_ppc_rev ttagccggtattacgcatacctgccgcaatcccggcaatagtgaccatatgaatatcctccttag (SEQ ID No. 164) pykA Δ_pykA_for atgtccagaaggcttcgcagaacaaaaatcgttaccacgttaggcgtgtaggctggagctgcttc (SEQ ID No. 165) Δ_pykA_rev ttactctaccgttaaaatacgcgtggtattagtagaacccacggtcatatgaatatcctccttag (SEQ ID No. 166) pykF Δ_pykF_for atgaaaaagaccaaaattgtttgcaccatcggaccgaaaaccgaagtgtaggctggagctgcttc (SEQ ID No. 167) Δ_pykF_rev ttacaggacgtgaacagatgcggtgttagtagtgccgctcggtaccatatgaatatcctccttag (SEQ ID No. 168) mgsA Δ_mgsA_for atggaactgacgactcgcactttacctgcgcggaaacatattgcggtgtaggctggagctgcttc (SEQ ID No. 169) Δ_mgsA_rev ttacttcagacggtccgcgagataacgctgataatcggggatcagcatatgaatatcctccttag (SEQ ID No. 170) iclR Δ_iclR_for atggtcgcacccattcccgcgaaacgcggcagaaaacccgccgttgtgtaggctggagctgcttc (SEQ ID No. 171) Δ_iclR_rev tcagcgcattccaccgtacgccagcgtcacttccttcgccgctttcatatgaatatcctccttag (SEQ ID No. 172) icd Δ_icd_for atggaaagtaaagtagttgttccggcacaaggcaagaagatcaccgtgtaggctggagctgcttc (SEQ ID No. 173) Δ_icd_rev ttacatgttttcgatgatcgcgtcaccaaactctgaacatttcagcatatgaatatcctccttag (SEQ ID No. 174) sucA Δ_sucA_for atgcagaacagcgctttgaaagcctggttggactcttcttacctcgtgtaggctggagctgcttc (SEQ ID No. 175) Δ_sucA_rev ttattcgacgttcagcgcgtcattaaccagatcttgttgctgtttcatatgaatatcctccttag (SEQ ID No. 176) sucB Δ_sucB_for atgagtagcgtagatattctggtccctgacctgcctgaatccgtagtgtaggctggagctgcttc (SEQ ID No. 177) Δ_sucB_rev ctacacgtccagcagcagacgcgtcggatcttccagcaactctttcatatgaatatcctccttag (SEQ ID No. 178) frdA Δ_frdA_for gtgcaaacctttcaagccgatcttgccattgtaggcgccggtggcgtgtaggctggagctgcttc (SEQ ID No. 179) Δ_frdA_rev tcagccattcgccttctccttcttattggctgcttccgccttatccatatgaatatcctccttag (SEQ ID No. 180) frdB Δ_frdB_for atggctgagatgaaaaacctgaaaattgaggtggtgcgctataacgtgtaggctggagctgcttc (SEQ ID No. 181) Δ_frdB_rev ttagcgtggtttcagggtcgcgataagaaagtctttcgaactttccatatgaatatcctccttag (SEQ ID No. 182) frdC Δ_frdC_for atgacgactaaacgtaaaccgtatgtacggccaatgacgtccaccgtgtaggctggagctgcttc (SEQ ID No. 183) Δ_frdC_rev ttaccagtacagggcaacaaacaggattacgatggtggcaaccaccatatgaatatcctccttag (SEQ ID No. 184) frdD Δ_frdD_for atgattaatccaaatccaaagcgttctgacgaaccggtattctgggtgtaggctggagctgcttc (SEQ ID No. 185) Δ_frdD_rev ttagattgtaacgacaccaatcagcgtgacaactgtcaggatagccatatgaatatcctccttag (SEQ ID No. 186) ptsI Δ_ptsI_for atgatttcaggcattttagcatccccgggtatcgctttcggtaaagtgtaggctggagctgcttc (SEQ ID No. 187) Δ_ptsI_rev ttagcagattgttttttcttcaatgaacttgttaaccagcgtcatcatatgaatatcctccttag (SEQ ID No. 188) ptsG Δ_ptsG_for atgtttaagaatgcatttgctaacctgcaaaaggtcggtaaatcggtgtaggctggagctgcttc (SEQ ID No. 189) Δ_ptsG_rev ttagtggttacggatgtactcatccatctcggttttcaggttatccatatgaatatcctccttag (SEQ ID No. 190) lacI Δ_lacI_for gtgaaaccagtaacgttatacgatgtcgcagagtatgccggtgtcgtgtaggctggagctgcttc (SEQ ID No. 191) Δ_lacI_rev tcactgcccgctttccagtcgggaaacctgtcgtgccagctgcatcatatgaatatcctccttag (SEQ ID No. 192) lldD Δ_lldD_for atgattatttccgcagccagcgattatcgcgccgcagcgcaacgcgtgtaggctggagctgcttc (SEQ ID No. 193) Δ_lldD_rev ctatgccgcattccctttcgccatgggagccagtgccgcaggcaacatatgaatatcctccttag (SEQ ID No. 194) pgi Δ_pgi_for atgaaaaacatcaatccaacgcagaccgctgcctggcaggcactagtgtaggctggagctgcttc (SEQ ID No. 195) Δ_pgi_rev ttaaccgcgccacgctttatagcggttaatcagaccattggtcgacatatgaatatcctccttag (SEQ ID No. 196) metA Δ_metA_for atgccgattcgtgtgccggacgagctacccgccgtcaatttcttggtgtaggctggagctgcttc (SEQ ID No. 197) Δ_metA_rev ttaatccagcgttggattcatgtgccgtagatcgtatggcgtgatcatatgaatatcctccttag (SEQ ID No. 198) thrB Δ_thrB_for atggttaaagtttatgccccggcttccagtgccaatatgagcgtcgtgtaggctggagctgcttc (SEQ ID No. 199) Δ_thrB_rev ttagttttccagtactcgtgcgcccgccgtatccagccggcaaatcatatgaatatcctccttag (SEQ ID No. 200) lysA Δ_lysA_for atgccacattcactgttcagcaccgataccgatctcaccgccgaagtgtaggctggagctgcttc (SEQ ID No. 201) Δ_lysA_rev ttaaagcaattccagcgccagtaattcttcgatggtctggcgacgcatatgaatatcctccttag (SEQ ID No. 202) eda Δ_eda_for atgaaaaactggaaaacaagtgcagaatcaatcctgaccaccggcgtgtaggctggagctgcttc (SEQ ID No. 203) Δ_eda_rev ctcgatcgggcattttgacttttacagcttagcgccttctacagccatatgaatatcctccttag (SEQ ID No. 204) recA Δ_recA_for atggctatcgacgaaaacaaacagaaagcgttggcggcagcactggtgtaggctggagctgcttc (SEQ ID No. 205) Δ_recA_rev ttaaaaatcttcgttagtttctgctacgccttcgctatcatctaccatatgaatatcctccttag (SEQ ID No. 206) asd Δ_asd_for atgaaaaatgttggttttatcggctggcgcggtatggtcggctccgtgtaggctggagctgcttc (SEQ ID No. 207) Δ_asd_rev ttacgccagttgacgaagcatccgacgcagcggctccgcggcccccatatgaatatcctccttag (SEQ ID No. 208)

TABLE 11 Primer pairs used for verification of gene disruptions Deleted Sequence (5′ - 3′) gene Forwardprimer Reverse primer K2 for/ cggtgccctgaatgaactgc cagtcatagccgaatagcct k1 rev (SEQ ID No. 209) (SEQ ID No. 210) ldhA atacgtgtcccgagcggtag tacacatcccgccatcagca (SEQ ID No. 211) (SEQ ID No. 212) adhE Gaagtaaacgggaaaatcaa Agaagtggcataagaaaacg (SEQ ID No. 213) (SEQ ID No. 214) ackA ccattggctgaaaattacgc gttccattgcacggatcacg (SEQ ID No. 215) (SEQ ID No. 216) focA_pflB atgccgtagaagccgccagt tgttggtgcgcagctcgaag (SEQ ID No. 217) (SEQ ID No. 218) pta gcaaatctggtttcatcaac tcccttgcacaaaacaaagt (SEQ ID No. 219) (SEQ ID No. 220) poxB ggatttggttctcgcataat agcattaacggtagggtcgt (SEQ ID No. 221) (SEQ ID No. 222) sad gctgattctcgcgaataaac aaaaacgttcttgcgcgtct (SEQ ID No. 223) (SEQ ID No. 224) gabD tctgtttgtcaccaccccgc Aagccagcacctggaagcag (SEQ ID No. 225) (SEQ ID No. 226) gadA aagagctgccgcaggaggat gccgccctcttaagtcaaat (SEQ ID No. 227) (SEQ ID No. 228) gadB ggattttagcaatattcgct cctaatagcaggaagaagac (SEQ ID No. 229) (SEQ ID No. 230) gadC gctgaactgttgctggaaga ggcgtgcttttacaactaca (SEQ ID No. 231) (SEQ ID No. 232) sfcA tagtaaataacccaaccggc tcagtgagcgcagtgtttta (SEQ ID No. 233) (SEQ ID No. 234) maeB attaatggtgagagtttgga tgcttttttttattattcgc (SEQ ID No. 235) (SEQ ID No. 236) ppc gctttataaaagacgacgaa gtaacgacaattccttaagg (SEQ ID No. 237) (SEQ ID No. 238) pykA tttatatgcccatggtttct atctgttagaggcggatgat (SEQ ID No. 239) (SEQ ID No. 240) pykF ctggaacgttaaatctttga ccagtttagtagctttcatt (SEQ ID No. 241) (SEQ ID No. 242) iclR gatttgttcaacattaactcatcgg tgcgattaacagacaccctt (SEQ ID No. 243) (SEQ ID No. 244) mgsA tctcaggtgctcacagaaca tatggaagaggcgctactgc (SEQ ID No. 245) (SEQ ID No. 246) icd cgacctgctgcataaacacc tgaacgctaaggtgattgca (SEQ ID No. 247) (SEQ ID No. 248) sucA acgtagacaagagctcgcaa catcacgtacgactgcgtcg (SEQ ID No. 249) (SEQ ID No. 250) sucB tgcaactttgtgctgagcaa tatcgcttccgggcattgtc (SEQ ID No. 251) (SEQ ID No. 252) frdA Aaatcgatctcgtcaaatttcagac aggaaccacaaatcgccata (SEQ ID No. 253) (SEQ ID No. 254) frdB gacgtgaagattactacgct agttcaatgctgaaccacac (SEQ ID No. 255) (SEQ ID No. 256) frdC tagccgcgaccacggtaagaaggag cagcgcatcacccggaaaca (SEQ ID No. 257) (SEQ ID No. 258) frdD atcgtgatcattaacctgat ttaccctgataaattaccgc (SEQ ID No. 259) (SEQ ID No. 260) ptsG ccatccgttgaatgagtttt tggtgttaactggcaaaatc (SEQ ID No. 261) (SEQ ID No. 262) ptsI gtgacttccaacggcaaaag ccgttggtttgatagcaata (SEQ ID No. 263) (SEQ ID No. 264) lacI Gaatctggtgtatatggcga Tcttcgctattacgccagct (SEQ ID No. 265) (SEQ ID No. 266) lldD Cgtcagcggatgtatctggt Gcggaatttctggttcgtaa (SEQ ID No. 267) (SEQ ID No. 268) pgi Ttgtcaacgatggggtcatg Aaaaatgccgacataacgtc (SEQ ID No. 269) (SEQ ID No. 270) lysA Tctcaaagcgcgcaagttcg Ggtattgatgtaccgggtgagatt (SEQ ID No. 271) (SEQ ID No. 272) metA Tcgacagaacgacaccaaat Cactgtgaacgaaggatcgt (SEQ ID No. 273) (SEQ ID No. 274) thrB Tgttggcaatattgatgaag Gacatcgctttcaacattgg (SEQ ID No. 275) (SEQ ID No. 276) eda Gacagacaggcgaactgacg Gcgcagatttgcagattcgt (SEQ ID No. 277) (SEQ ID No. 278) recA Tggcggcagtgaagagaagc Gcaataacgcgctcgtaatc (SEQ ID No. 279) (SEQ ID No. 280) asd Acaaagcaggataagtcgca Gacttcaggtaaggctgtga (SEQ ID No. 281) (SEQ ID No. 282) rhtA CAGAGAACTGCGTAAGTATTACGCA TAGTGGTAACAAGCGTGAAAAACAA (SEQ ID No. 283) (SEQ ID No. 284) rhtB ATGAAGACTCCGTAAACGTTTCCCC CAAAAATAGACACACCGGGAGTTCA (SEQ ID No. 285) (SEQ ID No. 286)

The plasmid co-expressing aspartate kinase, aspartate semialdehyde dehydrogenase, and homoserine dehydrogenase (pACT3-op-HMS1) was transformed together with the plasmid expressing the homoserine transaminase and the OHB reductase (pEXT2O-DHB) into the optimized host strains. Transformants were selected on solid LB medium containing chloramphenicol (25 μg/mL) and ampicillin (100 μg/mL). Non-exclusive examples of constructed strains are listed in Table 12.

TABLE 12 Examples of strains constructed for DHB production Strain Relevant Genotype MG1655 Wild-type ECE73 ΔldhA ΔadhE ΔmetA ΔthrB ECE74 ΔldhA ΔadhE ΔmetA ΔthrB pACT3-op-HMS1 ECE75 ΔldhA ΔadhE ΔmetA ΔthrB pEXT20-DHB ECE76 ΔldhA ΔadhE ΔmetA ΔthrB pACT3-op-HMS1 pEXT20-DHB ECE77 ΔldhA ΔadhE ΔmetA ΔthrB ΔlldD pACT3-op-HMS1 pEXT20-DHB ECE78 ΔldhA ΔadhE ΔmetA ΔthrB ΔrhtB pACT3-op-HMS1 pEXT20-DHB

It is understood that removal of the lacI gene from the backbone of the above described plasmids along with the genomic deletion of lacI in the host strain may render protein expression from above described plasmids constitutive.

Example 9 Demonstration of the Zymotic Production of DHB Via the Homoserine-OHB Pathway

Strains and cultivation conditions: Experiments were carried out with strains listed in Table 12. All cultivations were carried out at 37° C. on an Infors rotary shaker running at 170 rpm. Overnight cultures (3 mL medium in test tube) were inoculated from glycerol stocks and used to adjust an initial OD₆₀₀ of 0.05 in 100 mL growth cultures cultivated in 500 mL shake flasks. IPTG was added at a concentration of 1 mmol/L when OD₆₀₀ in the growth cultures reached 0.8. One liter culture medium contained, 20 g glucose, 18 g Na₂HPO₄*12 H₂O, 3 g KH₂PO₄, 0.5 g NaCl, 2 g NH₄Cl, 0.5 g MgSO₄*7 H₂O, 0.015 CaCl₂*2 H₂O, 1 mL of 0.06 mol/L FeCl₃ stock solution prepared in 100 times diluted concentrated HCl, 2 mL of 10 mM thiamine HCl stock solution, 20 g MOPS and 1 mL of trace element solution (containing per liter: 0.04 g Na₂EDTA*2H₂O, 0.18 g CoCl₂*6 H₂O, ZnSO4*7 H₂O, 0.04 g Na₂MoO4*2 H₂O, 0.01 g H₃BO₃, 0.12 g MnSO₄*H₂O, 0.12 g CuCl₂*H2O.). Medium pH was adjusted to 7 and medium was filter-sterilized. The antibiotics kanamycin sulphate, ampicillin, and chloramphenicol were added at concentrations of 50 mg/L, 100 mg/L, and 25 mg/L, respectively, when necessary.

Estimation of DHB concentration by LC-MS analyses: Liquid anion exchange chromatography was performed on an ICS-3000 system from Dionex (Sunnyvale, USA) equipped with an automatic eluent (KOH) generator system (RFIC, Dionex), and an autosampler (AS50, Dionex) holding the samples at 4° C. Analytes were separated on an lonPac AS11 HC (250×2 mm, Dionex) column protected by an AG11 HC (50×2 mm, Dionex) pre-column. Column temperature was held at 25° C., flow rate was fixed at 0.25 mL/min, and analytes were eluted applying the KOH gradient described earlier (Groussac E, Ortiz M & Francois J (2000): Improved protocols for quantitative determination of metabolites from biological samples using high performance ionic-exchange chromatography with conductimetric and pulsed amperometric detection. Enzyme. Microb. Technol. 26, 715-723). Injected sample volume was 15 μL. For background reduction, an ASRS ultra II (2 mm, external water mode, 75 mA) anion suppressor was used. Analytes were quantified using a mass-sensitive detector (MSQ Plus, Thermo) running in ESI mode (split was ⅓, nitrogen pressure was 90 psi, capillary voltage was 3.5 kV, probe temperature was 450° C.).

Results:

After 24 h cultivation, the DHB concentration in the supernatant of different strains was quantified by LC-MS analyses. The strains ECE73, ECE74, ECE75, and ECE76 had produced 0 mg/L, 3.7 mg/L, 0.67 mg/L, and 11.9 mg/L of DHB, respectively.

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1-22. (canceled)
 23. A method for the preparation of 2,4-dihydroxybutyrate (2,4-DHB) from homoserine comprising: deaminating homoserine to form 2-oxo-4-hydroxybutyrate (OHB), where the deamination of homoserine is catalyzed by an enzyme having homoserine transaminase activity, wherein the enzyme having homoserine transaminase activity is produced via a transformed host microorganism that comprises a first chimeric gene including a first nucleic acid sequence encoding the enzyme having homoserine transaminase activity for converting the primary amino acid group of homoserine to a carbonyl group to obtain OHB; and reducing the OHB to form 2,4-DHB, where the reduction of OHB is catalyzed by an enzyme having OHB reductase activity, wherein the enzyme having OHB reductase activity is produced via the transformed host microorganism, which further comprises a second chimeric gene including a second nucleic acid sequence encoding the enzyme having OHB reductase activity for reducing OHB to 2,4-DHB.
 24. The method of claim 23, wherein the enzyme having homoserine transaminase activity is selected from the group consisting of enzymes classified in E.C. 2.6.1.1, E.C. 2.6.1.2, E.C. 2.6.1.42, E.C. 2.6.1.57 or E.C. 2.6.1.88.
 25. The method of claim 24, wherein the enzyme having homoserine transaminase activity is selected from: a transaminase having a sequence SEQ ID NO: 64 or encoded by the gene aspC, a transaminase having a sequence SEQ ID NO: 60 or encoded by the gene ilvE, a transaminase having a sequence SEQ ID NO: 68 or encoded by the gene bcaT, a transaminase having a sequence SEQ ID NO: 62 or encoded by the gene tyrB, a transaminase having a sequence SEQ ID NO: 66 or encoded by the gene araT, a transaminase having a sequence SEQ ID NO: 70 or encoded by the gene ARO8, a transaminase encoded by the gene alaC, a transaminase encoded by the gene mtnE, a transaminase encoded by the gene ybdL; or is selected from any sequence sharing a sequence identity of at least 90% with at least one of the sequences of said enzymes.
 26. The method of claim 23, wherein the enzyme having OHB reductase activity is selected from the group consisting of lactate dehydrogenases classified in E.C.1.1.1.27 or E.C.1.1.1.28, malate dehydrogenases classified in E.C.1.1.1.37, E.C.1.1.1.82 or E.C.1.1.1.299, or branched-chain 2-hydroxyacid dehydrogenases classified in E.C.1.1.1.272 or E.C.1.1.1.345.
 27. The method of claim 26, wherein the enzyme having OHB reductase activity is selected from the group consisting of SEQ ID NO: 2, SEQ ID NO: 4, SEQ ID NO: 6, SEQ ID NO: 8, SEQ ID NO: 10, SEQ ID NO: 12, SEQ ID NO: 14, SEQ ID NO: 288, SEQ ID NO: 30, SEQ ID NO: 32, SEQ ID NO: 102, SEQ ID NO: 104, SEQ ID NO: 106, SEQ ID NO: 108, SEQ ID NO: 110, SEQ ID NO: 112, SEQ ID NO: 114, SEQ ID NO: 116 or SEQ ID NO: 118, or is selected from any sequence sharing a sequence identity of at least 90% with at least one of said sequences.
 28. The method of claim 27, wherein the enzyme having OHB reductase activity is selected from the group consisting of (D)-lactate dehydrogenase from Escherichia coli (SEQ ID NO: 4), (L)-lactate dehydrogenase from Lactococcus lactis (SEQ ID NO: 6), the two isoforms of (L)-lactate dehydrogenase from Oryctalagus cuniculus (SEQ ID NO: 12 and SEQ ID NO: 14), (L)-lactate dehydrogenase from Geobacillus stearothermophilus (SEQ ID NO: 10), (L)-lactate dehydrogenase from Bacillus subtilis (SEQ ID NO: 8), (L)-malate dehydrogenase from Escherichia coli (SEQ ID NO: 2), branched chain (D)-2-hydroxyacid dehydrogenase from Lactococcus lactis, and dehydrogenases having an amino acid sequence sharing a sequence identity of at least 90% with at least one of said sequences.
 29. The method of claim 28, wherein the enzyme having OHB reductase activity is a lactate dehydrogenase comprising at least one mutation in position V17, Q85, E89, 1226, or A222, said positions being defined by reference to the L-Lactis LdhA (SEQ. ID NO: 6); or a malate dehydrogenase comprising at least one mutation in position A112, R81, M85, D86, V93, G179, T211, or M227 said positions being defined by reference to the E. coli Mdh (SEQ ID NO: 2).
 30. The method of claim 23, wherein the enzyme having homoserine transaminase activity is selected from the group consisting of enzymes classified in E.C. 2.6.1.1, E.C. 2.6.1.2, E.C. 2.6.1.42, E.C. 2.6.1.57 or E.C. 2.6.1.88, and wherein the enzyme having OHB reductase activity is a lactate dehydrogenase classified in E.C.1.1.1.27 or E.C.1.1.1.28, a malate dehydrogenase classified in E.C.1.1.1.37, E.C.1.1.1.82 or E.C.1.1.1.299, or a branched-chain 2-hydroxyacid dehydrogenase classified in E.C.1.1.1.272 or E.C.1.1.1.345.
 31. A modified microorganism for the preparation of 2,4-dihydroxybutyrate (2,4-DHB) from homoserine via a two-step pathway comprising: deaminating homoserine to form 2-oxo-4-hydroxybutyrate (OHB), where the deamination of homoserine is catalyzed by an enzyme having homoserine transaminase activity, and reducing the OHB to form 2,4-DHB, where the reduction of OHB is catalyzed by an enzyme having OHB reductase activity; wherein the modified microorganism is a host microorganism that has been transformed to enhance production of 2,4-DHB compared to a non-transformed host microorganism, the transformed host microorganism comprising: a first chimeric gene including a first nucleic acid sequence encoding the enzyme having homoserine transaminase activity for converting the primary amino acid group of homoserine to a carbonyl group to obtain OHB, and a second chimeric gene including a second nucleic acid sequence encoding the enzyme having OHB reductase activity for reducing OHB in 2,4-DHB.
 32. The modified microorganism of claim 31, wherein the transformed host microorganism has been further transformed to enhance production of homoserine compared to the non-transformed host microorganism.
 33. The modified microorganism of claim 32, wherein the enhanced production of homoserine comprises overexpressing one or more additional enzymes selected from the group consisting of aspartate kinase, aspartate semialdehyde dehydrogenase and homoserine dehydrogenase, wherein the overexpression of said one or more enzymes is realized by expressing the enzymes from a multicopy plasmid.
 34. The modified microorganism of claim 31, wherein the modified microorganism is a bacterium, a yeast, or a fungus.
 35. The modified microorganism of claim 31, wherein the expression of at least of one the enzymatic activities chosen among phosphoenolpyruvate carboxylase, phosphoenolpyruvate carboxykinase, isocitrate lyase, pyruvate carboxylase, and hexose symporter permease is increased, and/or at least one of the enzymatic activities chosen among lactate dehydrogenase, alcohol dehydrogenase, acetate kinase, phosphate acetyltransferase, pyruvate oxidase, isocitrate lyase, fumarase, 2-oxoglutarate dehydrogenase, pyruvate kinase, malic enzyme, phosphoglucose isomerase, phosphoenolpyruvate carboxylase, phosphoenolpyruvate carboxykinase, pyruvate-formate lyase, succinic semialdehyde dehydrogenase, sugar-transporting phosphotransferase, ketohydroxyglutarate aldolase, homoserine-O-succinyl transferase, homoserine kinase, homoserine efflux transporter, diaminopimelate decarboxylase, and/or methylglyoxal synthase is decreased.
 36. The modified microorganism of claim 34, the modified microorganism being Escherichia coli, which overexpresses at least one of the genes chosen among ppc (phosphoenol pyruvate carboxylase), pck, aceA, galP, asd, thrA, metL, lysC all E coli; pycA from L lactis, and/or has at least one of the genes deleted chosen among IdhA, adhE, ackA, pta, poxB, focA, pfIB, sad, gabABC, sfcA, maeB, ppc, pykA, pykF, mgsA, sucAB, ptsl, ptsG, pgi, fumABCaldA, HdD, icIR, metA, thrB, lysA, eda, rthA, rthB, and rthC.
 37. The modified microorganism of claim 31, wherein the enzyme having homoserine transaminase activity is selected from the group consisting of enzymes classified in E.C. 2.6.1.1, E.C. 2.6.1.2, E.C. 2.6.1.42, E.C. 2.6.1.57 or E.C. 2.6.1.88, and/or wherein the enzyme having OHB reductase activity is a lactate dehydrogenase classified in E.C.1.1.1.27 or E.C.1.1.1.28, a malate dehydrogenase classified in E.C.1.1.1.37, E.C.1.1.1.82 or E.C.1.1.1.299, or a branched-chain 2-hydroxyacid dehydrogenase classified in E.C.1.1.1.272 or E.C.1.1.1.345.
 38. A method of production of 2,4-DHB comprising the steps of culturing the modified microorganism of claim 31 in an appropriate culture medium, recovering 2,4-DHB from the culture medium.
 39. The method of claim 38 wherein the 2,4-DHB is further purified. 